The objective of this study is to investigate the removal mechanism of arsenite on ferrihydrite and construct the quantitative model. The adsorption and co-precipitation experiments were performed at pH 7. Based on x-ray absorption fine structure (XAFS) analysis in As K-edge, the arsenite was not oxidized during both adsorption and co-precipitation processes. The fact indicated that arsenite was removed by ferrihydrite as arsenite. Both adsorption and co-precipitation processes gave an unsaturated isotherm like Brunauer-Emmett-Teller type. It was suggested that the removal mechanism of arsenite on ferrihydrite in adsorption and co-precipitation processes was not only surface complexation but also another uptakes mechanism. From XAFS analysis in Fe K-edge, it revealed that the atomic distance between the central ferric atom and the nearest neighbor ferric / oxygen atom was expended as the initial As/Fe molar ratio increased in co-precipitation process. It indicated that arsenite was incorporated into the structure of ferrihydrite when the initial As/Fe molar ratio was 5.0. From these results, the main removal mechanism was surface complexation, but the phenomena that arsenite was incorporated into the structure of ferrihydrite was also involved. Based on removal mechanism investigations, a quantitative model using diffuse layer model was constructed. To construct a quantitative model, the exchange capacity and surface complexation coefficients for H+, OH- were set at values proposed by Dzombak and Morel . The surface complexation coefficients for only As(III) adsorption on ferrihydrite was determined by fitting to the experimental adsorption results. The experimental results for As(III) removal by ferrihydrite in the adsorption process was successfully represented by our constructed model.