Abstract: In present study, drinking surface water of different dams, located in Rawalpindi / Islamabad region, was analyzed. Water samples were observed bacteriologically for total viable count and total coliform count by heterotrophic plate count and MPN method. Pathogenic bacteria were isolated from water samples collected from different dams and related filtration plants and identified by different bio-chemical tests. Water samples contain different pathogenic bacteria like Bacillus cereus, E. aerogens, S. aureus, E. coli, Salmonella spp., Shigella spp. and Streptobacillus spp. In all filtration plants, chlorine, a sterilizing agent and the main co-agulant, Alum sulphate, was used. Growth of Coliforms was observed after chlorination in water samples, collected from Rawal dam filtration plant. While no growth of any microorganisms was observed after chlorination in case of Simly dam and Khanpur dam filtration plants but water sample taken from storage of Khanpur filtration plant after filtration and chlorination, was also contaminated. The objective of this study was to check the quality of drinking water and comparative analysis of different dams (Rawal, Simly and Khanpur) and related filtration plants which supplying water for drinking and other purposes to different inhabitants of Rawalpindi and Islamabad after filtration and chlorination.