Drinking water supply organisation in Tehran has 24 chlorination stations. These are located randomly between houses across the city. Tehran is a crowded city with high density of population, 11,360.9 persons in every km2. There are large numbers of people at risk if chlorine gas is accidentally released in these stations. The aim of study was evaluation of chlorine gas release hazard in the chlorination stations. To achieve this goal, FTA, a popular safety analysis method, was used. The identified basic events (BE)-a total of 1,224-were categorised into four groups: human errors (HE), management oversights (MO), design errors (DE), and hardware failures (HF). Distribution of BEs in these groups showed that the highest percent of them were in the HE group (43.5%), 31.6 % in the MO, 24.1% in the DE, and 0.8 % in the HF group. Quality analysis on the fault tree yielded a total of 2,083 Minimal Cut Sets. The probability of chlorine gas being accidentally released in the stations varied from 10 to 21/yr. In chlorinator room this probability was 1.43 times higher than in the cylinder room. In prevention of a chlorine gas release accident, lack of training has a very important role and the design of stations is more important than the design of the equipment.