The objective of the present study was to evaluate rain water quality of Khorramabad for one year at 4 stations at 4 different points using some parameters and Multivariate statistics have been applied to reveal the primary factors that affect rain water quality, and the differences and relation among the studied water quality parameters in the samples. Total dissolved solids (TDS), pH, conductivity, turbidity, total hardness, carbonate and non carbonate hardness, magnesium and calcium hardness, calcium and magnesium cataions, Chemical oxygen demand(COD), Nitrate, biological quality using most probable number per 100 mL (total and fecal MPN/100ml) and IMVIC ((indole, methyl red, Voges-Proskauer, and citrate)) tests were analyzed by standard analytical techniques. Based obtained results, the collected rainwater had acceptable physicochemical quality but did not meet the requirements for drinking water. Results showed that 62.5% and 50% of obtained samples had detectable values of total coliforms and fecal coliforms, respectively. Klebsiella pneumuniae was the widely detected bacteria as a result of IMViC test for the majority of samples. Extraction Sums of Squared Loadings for first, second and third components were 52.64%, 18.99% and 8.7% of variance respectively. To confirm the associations between the variables in then total dataset, CA was performed on the measured variables, which showed adequate agreement between results obtained by unsupervised FA and CA to confirm the conclusions made over the complete dataset. It can be concluded these data sets can be valuable references for harvesting and managing of rain water, especially at studied area. Keywords: Rain water quality, Factor analysis, Cluster analysis, microbiological quality. 1. INTRODUCTION Today the water shortage is a serious and global problem. Growing population, indiscriminate use of water and continuing drought have led to water shortage in Iran. Furthermore, the problem of water shortage can intensify the problem of quality (at physicochemical and biological aspect) in water supply. Factors as urbanization of societies, anthropogenic impact on the environment and industrial activity get worse the quality of water and sometimes make it unsuitable for use as water supply. A good solution for solving mentioned problems may be rainwater catchment. By rainwater harvesting can collect and store precipitation from rooftops, surface or rock catchments by simple technique and instruments. Rain water harvesting have been used in Asia and Africa arise by very old civilizations within these regions and still serve as a major source of drinking water supply in rural areas. Iran is s a country in Western Asia and Middle East, lies between 25?? 00?? and 39?? 47?? N and 44?? 02?? and 63?? 20?? E. Iran has an area of 1,648,195 km2 that at view of area it is eighteenth largest country in the world and second largest country in the Middle East. Iran has twenty eight provinces that divided in 293 cities. Iran with total population of 77 million people has second and seventeen place in the Middle East (after Egypt) and world, respectively. Iran has variety whether with average 250mm/year precipitation, which is 30% of global average annual precipitation. As Iran is in the arid zone, about 65% of its territory has an arid or hyper arid climate, and approximately 85% of the country an arid, semi-arid or hyper arid environment. Based on available information underground water reserves in the Middle East are under threat. By current trends in water use of the region humans as well as the vast ecosystems that water supports, are blindly walking into crisis. Iranians people used most of their groundwater reserves. The groundwater situation has been reported to be extremely critical in some parts of the country. Figure 1 shows water stress world-wide, especially in Iran. Daily water consumption of Iranians is more than 250 l per capita and it can exceed 400 l. This means that is twice the world’s standard despite its limited water availability. Rain has sufficient quality at view of softness and purity, as well as it is a major component of the water cycle and is main source for depositing most of the fresh water on the Earth. Except polluted area (present polluting industries and polluted area geographically), rain water quality is better that raw surface water, at least at physicocemical aspect. Part of household uses such as gardening and plant irrigating, and non-potable uses can be supplied by harvested rain water without any treatment. In Iran Governments and Water Boards are going to increase water prices as they look to recover the true costs of providing water to the community, so using harvested rain water led to use less mains water and can reduce your water bills. E. coli (and fecal coliforms) is the most common microbial contamination of harvested rain water samples. Water quality is the key factor to rain water application developing. Leotsinidis and colleagues assessed the quality of harvested rainwater which is used for domestic and drinking purposes in the northern area of Kefalonia Island in SW Greece and the factors affecting it were assessed through 3-year surveillance. As far as microbiological quality is concerned, total coliforms, Escherichia coli and enterococci were detected in 80.3%, 40.9% and 28.8% of the rainwater samples, respectively, although they were found in low concentrations. Vialleet. al., The quality of harvested rainwater used for toilet flushing in a private house in the southwest of France was assessed over a one-year period. total coliforms, E. coli and enterococci ‘ were detected in the majority of samples, indicating microbiological contamination of the water. According to the results of RashidiMehrabadiet. al., in humid climate, it is possible to supply at least 75% of non-potable water demand by storing rainwater from larger roof areas for a maximum duration of 70% of the times. For roofs with small surface area, the supply meets 75% of non-potable water demand for a maximum duration of 45% of the times. Based on Radaidehet. al., study indicated that water quality in these tanks and cisterns varies depending on location, on ment area, and on the availability of public sanitary systems. It was concluded that collected rainwater is unsuitable foring purpose while it could be used for irrigation within in houses. In Iran, Khorammabad governmental agencies monitor public water resources continuously for physico chemical and biological parameters. Unfortunately, In Iran there is no interest in rain water as a water resource, so there is no program for rain water quality monitoring. This study was conducted to monitor physicochemical and microbiological parameters of collected rain water and to use factor and cluster analysis to further characterize associations present in the complete data set. Rainwater was collected over a one-year period simultaneous with raining which sample be enough for analyzing. In the present study, unlike similar studies, we analyzed collected rain water samples without flowing on roof, which can show real rain water quality without roof interference.