Synthetic samples of different grain size fractions of quartz were prepared and the thermal conductivities measured with a divided bar apparatus. The measurements were carried out on dried and water-saturated samples both perpendicular and parallel to the main stress direction during compaction of the samples. A clear correlation was observed between the porosity and the measured thermal conductivities. For water-saturated samples the thermal conductivity decreases more with increasing porosity than for ideal isotropic material as predicted by the geometric mean model. A clear positive correlation was also found between the thermal conductivity and the grain sizes of the samples. When the expected effect of porosity on the thermal conductivity was excluded, a linear correlation between the logarithm of the grain size and the measured thermal conductivities of the water-saturated samples was found. An anisotropy effect (a = k/k 1 ) in thermal conductivity was observed for the samples, being most prominent for the coarsest ones. The measured anisotropy is assumed to be mainly due to the thermal anisotropic nature of the quartz crystal.