Thermal conductivity of simulated high-level radioactive waste glass was measured by a radial heat flow technique at temperatures from 300 to 1250 K, using two types of cell. Below glass transition temperature Tg (720 K), the thermal conductivity was determined to be In an attempt to clarify the mechanism of heat transfer in waste glass, the radiative thermal conductivity was determined using the absorption coefficient of photons in the waste glass. The measured thermal conductivity was compared with the radiative thermal conductivity and behavior of heat capacity. It was determined that (a) at temperatures above 1000 or 1100K, thermal conductivity included thermal radiation (radiative conduction) by a factor of 0.1 to 0.2 and (b) at temperatures above 1200 K, thermal conductivity seemed to be influenced by the scattering of photons by immiscible phases such as pores and inclusions.