Abstract: For selective separation of ¹³⁷Cs from high level liquid wastes(HLLWs), a silica based ammonium molybdophosphate(AMP/SiO₂) inorganic adsorbent was synthesized by combining the rotary vacuum evaporation method with pore crystallization. The effect of γ-ray, temperature, coexisting ions on the adsorption performance toward Cs(Ⅰ), and chemical stability of AMP/SiO₂ was also investigated. The adsorbent, with particle diameter of 90 μm and pore size of 18 nm, has characteristics of nano materials. The obvious decomposition of the AMP/SiO₂ occurres about 400 ℃ resulted the decreasing of the (2 2 0) diffraction peak intensity and formation of MoO₃. Gamma irradiation causes the transformation of some AMP from crystalline phase to amorphous phase and has less effect on adsorption capacity on Cs(Ⅰ). After 100 kGy absorbed dose, the average grain size of AMP is estimated to be 383 Å(1 Å=0.1 nm) and corresponds to be a cube cell with 11.6 side length. The AMP/SiO₂ has relative high selectivity toward Cs(Ⅰ) in the range of 0.1-7.0 mol/L HNO₃ simulated liquid waste, which follows Langmuir model and ion exchange process.