Background & Objectives: Trihalomethanes in the form of chloroform, bromodichloromethane (BDCM), dibromochloromethane (DBCM), and bromoform.are a group of compounds that can form when the chlorine used to disinfect drinking water reacts with naturally occurring organic matter. Potential hazards and carcinogenic characteristics of THMs have already been recognized. Therefore, current study attempted to measure concentration of these compounds in output (drinking water) of Jlalieh water treatment plant. Methods: In this study concentrations of trihalomethanes with chloroform index have been measured in the output of Jalalieh water treatment plant in 2009. Monthly water sampling carried out in morning. Samples were taken to the lab and sodium thiosulfate was used for de-chlorination of the water samples. Samples were analyzed in a time span of less than 6 hours using a GC set. Results: The average concentration of chloroform in input crude water of Jalalieh water treatment plant was equal to 7.42 µg/l. The average pH value in input and output of water treatment plant was equal to 7.92 , and 7.87, respectively. Conclusions: The average concentration of chlorine in input and output of water treatment plant was equal to 0.73 mg/l, and 0.94 mg/l, respectively. Although concentration of free chlorine was 0.94 mg/l in output treated water, however, the chloroform concentration in output treated water was lower than the permissible value (200 µg/l) recommended by WHO for drinking water. The highest concentration of THMs was found in April.