In order to understand the microstructure and thermophysical properties of Ag-In-Cd alloy in reactor under irradiation, the Ag-In-Cd-Sn alloy specimens were fabricated with the same composition for a given neutron irradiation condition. The microstructure, thermal diffusivity, specific heat, thermal conductivity and thermal expansion have been investigated by optical microscope (OM), scanning electron microscope (SEM), X ray diffractometer (XRD) and differential methods, differential scanning calorimeter, laser bombard method. The results show that the transformation induced by chemical modifications inside the single phase (FCC) alloy and, further, formation of two phases (FCC HCP). The second phase was Ag3In. The thermal conductivity, specific heat, thermal diffusivity and thermal expansion of Ag-In-Cd-Sn alloy increased as the temperature increased under the uniform Sn content between 25 to 600℃. However, the thermal conductivity and thermal diffusivity decreased obviously but the specific heat and thermal expansion increased appreciably as the Ag content decreased and the Sn content increased respectively.