In this study, the removal and transport mechanisms of ionized and non-ionized arsenics through NF and UF membranes were systemically investigated. The charge repulsion between the membrane surface and arsenic ions was an important mechanism for the rejection of ions by a charged membrane. In addition, the effect of J0/k ratio was dependent on the membrane and ion charge, but the cross-flow velocity was not significantly affected. Both diffusion and convection are proved to affect the transport of arsenic ions. The reflection coefficients (σ) of both UF and NF membranes increased with increasing pH; the reflection coefficients of arsenate were higher than those of arsenite under the same operating conditions. The spiral-wound module exhibited slightly higher arsenate removal than the flat-sheet module under the same operating conditions.