Arsenic contamination in groundwater has posed severe health problems to most of countries in the world. Nowadays, many methods of arsenic removal have been studied, but most of these are costly and impractical to be implemented in developing countries such as Cambodia. For this reason, the aim of this study is to determine the optimum concentration of iron supplying for high removal efficiency of As with three different conditions of variation ferric chloride adding. Arsenic contaminated groundwater samples known  through literature review were collected from Koh Thom district in Kandal province. The processes coke-bed trickling filter and sedimentation were conducted to remove arsenic from contaminated groundwater under three different conditions. Three conditions were chosen in the study. Condition A: no iron adding, Condition B: 25 mg/L of ferric chloride added into influent groundwater, Condition C: 50 mg/L of ferric chloride added to effluent from coke-bed within the same flow rate of 1L/day pumped from groundwater. This process used coke as the oxidation agent and ferric chloride as the coagulation agent to remove both of As(V) and As(III) from groundwater. The result shows that 25 mg/L dose of ferric chloride is good enough for high removal efficiency of total arsenic with the value 93.14 %. The residual concentration of arsenic could be brought down to 49.5 µg/L from initial concentration of 750 µg/L. However, the removal efficiency of total arsenic for condition A and C can achieve only 30% and 70% respectively. Base on the information above this system could remove As from As- contaminated groundwater on a small scale and at low cost operation. The system was considered as a small scale due to low amount of safe water obtained per day and as low cost operation due to installation with the simple materials such as plastic bottles, coke-bed, and others. This system is useful for people who have problem with arsenic contamination in water resource in Cambodia, especially for people living in rural areas.