Abstract: To analyze the effect of various factors (sand content, dry density and water content) on the thermal conductivity of Gaomiaozi (GMZ01) bentonite and sand-bentonite mixtures (GMZM), which are considered as the first choice for buffer/backfill materials for the high-level waste (HLW) disposal in China, a series of experiments are conducted. A thermal probe based on hot wire method is employed to measure the thermal conductivity. The test results are analyzed by several models. The analytic results show that the thermal conductivity of both GMZ01 and GMZM increases with the increase of dry density and water content; and for different dry densities (1.7~1.9 g/cm3), the thermal conductivity of GMZM increases with the increase of the sand content. The higher the dry density, the more obvious the effect of sand content. However, for a dry specimen, the thermal conductivity increases with the percentage of sand until 30% and turns to decrease for higher sand content. For the prediction of the thermal conductivity of GMZ01 bentonite, Kahr’s equation shows the highest accuracy, while for that of GMZM, the Fricke’s equation shows an advantage among others.