Since the surface water is one of the main potable water resources, the usage of chlorine as a disinfectant has increased. Consequently the production rate of disinfection by-products (DBPs) such as Trihalomethane (THM) compounds has grown dramatically. In this paper the THMS concentration changes in the Sangar Water Treatment Plant (SWTP) and Rasht Water Distribution System (RWDS) is presented. The duration of these monitoring tasks were 6 months in 2007 and samples were collected every 2 weeks. Water samples were collected from five locations at SWTP and RWDS. Some independent variables including Total Organic Carbon (TOC), pH, temperature, and residual chlorine were measured by Pearson method to find a relation between THMS formation and these variables. In the case of TOC, Pearson method showed a correlation of r = 0.8096 for SWTP and r = 0.3696 for RWDS between THM formation and TOC. Also the relationship for SWTP was r = 0.239 and r = 0.2336 for RWDS between THM formation and temperature. Correlation between THM formation and pH, Pearson method showed r = 0.4658 for SWTP and r = 0.3232 for RWDS. In the case of residual chlorine, Pearson method showed a relationship of r = 0.7354 for SWTP and r = 0.5623 for RWDS. Results proved a direct relation between THMS concentration and distance of chlorination injection points. The results showed that in SWTP, 42.7 percent of THM compounds were removed after sedimentation and filtration.