Systems that have big transmission line may have problem on changes of pressure in the distribution system. Because the increase in water age is dependent on the difference between the production and consumption rates, high residence time in pipes and storage duration in water tanks. The goal of this study is to analyze the effects residence time to water quality in large water distribution systems. According to the results in Ahwaz, bacterial growth (HPC) has a direct relationship with water age, and changing rate of the bacterial growth (HPC) and the distance travelled and residence time have been caused to increase water age, bacterial growth (HPC), chlorine consumption and hydraulic change. So bacterial growth (HPC) doesn’t have any relationship with THMs and Tol Coli form, but has a reversed relationship with residual chlorine nearly.