Comparable studies were conducted between chemical oxidation and microbial oxidation of ferrous sulfate by a mixed culture of Acidithiobacillus ferrooxidans in the presence of different concentrations of As(Ⅴ). Eh, pH and Fe2+ concentration were monitored periodically and the final precipitation was analyzed by X_ray diffraction (XRD) and SEM. The data obtained showed and chemical oxidation of Fe2+ was very slow with a final oxidizing ratio of <8% and As(Ⅴ) had no effect on Fe2+ oxidation chemically. Slight enhancement of Fe2+ oxidation was observed by 100 mg/ L As(Ⅴ) in the microbial system. Complete oxidation of Fe2+ could be reached in about 60 hours in the presence of A. ferrooxidans with 500 mg～1g/L As(Ⅴ). However, microbial Fe2+ oxidation was greatly inhibited by 4g/L As(Ⅴ) and about 106 hours were needed for complete oxidation. Initial mole ratios of 100 As/(As+S) affected the final solid phase and the crystallization of the precipitation. In the microbial system, typical symmetric peaks of jarostie were clearly distinguished in the precipitates with ≤ 1g/L As(Ⅴ) but the crystallization was decreasing with the increase of As(Ⅴ) concentration. Only amorphous solid was observed in the precipitate with 4g/L As(Ⅴ). Elemental mapping indicated that As was evenly distributed in the precipitates either by adsorption or by structural incorporation during the iron oxidation. These results provide important information for the treatment of As contamination in AMD regions.