Abstract: The diffusion behavior of Se(Ⅳ) (surrogate to ⁷⁹Se (Ⅳ)) in illite-montmorillonite mixture was investigated by through-diffusion method. The results show that the migration of Se(Ⅳ) is altered significantly by the illitization of montmorillonite. The effective diffusion coefficient of Se(Ⅳ) increases from 3.89×10^-11 m²/s to 18.1×10^-11 m²/s. When the illite-montmorillonite ratio is in the range of 1∶9 and 5∶5, D_e values have no obvious change. Whereas, D_e values increase significantly at illite montmorillonite ratio of 6∶4, indicating that the predominant composition of the mineral clay plays a leading role in Se(Ⅳ) diffusion. When the illite-montmorillonite ratio increases from 1∶9 to 6∶4, the rock capacity and distribution coefficient of Se(Ⅳ) decrease from 4.07 to 0.75 and from 22.7×10^-4 m³/kg to 1.95×10^-4 m³/kg, respectively. Both of them decrease with the increase of illite content. It demonstrates that the hindrance of illite-montmorillonite mixture turns worse due to the weaker sorption of Se(Ⅳ) on illite, leading the increase of Se(Ⅳ) migration in illite-montmorillonite mixture.