The investigation was carried out in lab-scale sewer system focusing on hydrogen sulfide control when ferric iron ［Fe (III)］ was added. Two lab-scale gravitational sewer systems fed with synthetic domestic wastewater were operated at room temperature. Concentration of S 2- and H2S in two sewer systems was measured firstly. Then the possible reactive mechanism of ferric dosage was proposed and mass balance of those was calculated. The results showed that S 2- concentration in effluent appeared no more than 1 mg/L and hydrogen sulfide in each slot was below 40 ppm averagely in ferric addition reactor, which were lower obviously than that in the control reactor. On the other side, batch tests exhibited about 44% ferric was converted into ironic phosphate, which had no effect on restraining the activity of sulfate-reducing bacteria. In a word, Fe 3+ dosage could inhibit producing S 2- and releasing H2S effectively although most of the Fe 3+ was converted into ironic phosphate.