Abstract: The abundant phosphate in a high level nuclear waste repository will influence the migration of radionuclides. Thus, the effect of phosphate on the diffusion behavior of Se(Ⅳ) in Gaomiaozi (GMZ) bentonite was investigated by through-diffusion method. The diffusion parameters like effective diffusion coefficient (D_e) and rock capacity factor (α) were determined by house-made code named Fitting for Diffusion Parameters. The D_e and α values were about (0.61-1.3)×10^-11 m²/s and 0.16-0.58, respectively. It shows that the concentration of phosphate has insignificant effect on Se(Ⅳ) diffusion. Under acidic condition, both of D_e and α increased. The apparent diffusion coefficient (D_a) value decreased because Se(Ⅳ) could be sorbed on the surface of bentonite. Since α   was less than the total porosity, it could not be sorbed on bentonite.