Aims: In this study, the effect of arsenite and arsenate removal in synthetic water by coagulation using ferric chloride (FeCl 3 ) as coagulant and chitosan as coagulant aid were investigated. b>Materials and Methods: A conventional jar test apparatus was used to evaluate the coagulation process. The effects of different conditions of pH (5.5-9), coagulant doses of FeCl 3 0-60 mg/l), Arsenic speciation (As (V) and As (III)), Chitosan as coagulant aid, and initial arsenic concentrations (0.2-2 mg/l) to Arsenic removal was investigated by considering the residual arsenic and iron after coagulation. Results: The finding showed that the optimum pH for FeCl 3 as a coagulant to As (V) and As (III) removal was 7. As (V) removal efficiency of 90% or higher by FeCl 3 alone were achieved for initial As (V) concentration 0.2-2 mg/l at optimum dosages (15-30 mg/l). In contrast with As (V) removal, about 60% of the As (III) were removed in comparable conditions. The optimal dosage of chitosan was determined to be 0.5 mg/l. When chitosan (0.5 mg/l) was used as coagulant aid with FeCl 3 , the efficiency removal was enhanced almost to 100% and 80% for As (V) and As (III) removal, respectively. The optimum dosage of FeCl 3 was reduced, especially at low initial concentration of arsenic. Conclusions: Chitosan as natural coagulant aid improved arsenic removal efficiency by coagulation process using FeCl 3 . This method can be used for regions with drinking water contaminated with initial arsenic concentration less than 1 000 μg/l.