This study intends to monitor and evaluate the THMs concentration in the 7 different water districts in the city of Tehran. THMs are the main disinfectant by products (DBP) of chlorination, which are suspected to be human carcinogens. Chlorination is used as the main disinfection process for the drinking water in Tehran, therefore; it is important to monitor and evaluate the THM’s concentration in Tehran’s drinking water. Samples were taken from twenty one locations across the city of Tehran in triplicates during the first week-end of April, May and June 2009 and analyzed for THMs using a Purge and Trap device coupled with a GC/MS instrument (EPA method 524.2). The data indicates that, the concentration of THMs are much higher in districts which are supplied mostly with surface water sources such as areas in the northern part of Tehran, compared to districts with well water as their main water source which are located mostly in the south and south west part of Tehran. The average concentration of chloroform, bromodichloromethane, dibromochloromethan and bromoform were 3.32 ppb, 2.82 ppb, 1.22 ppb and 0.16 ppb respectively. The observed THMs concentrations do not seem to cause a major adverse health effect on the population of Tehran, since the total THMs concentration is below the EPA’s recommended MCL value of 80 ppb.