Heat transfer is a key process for the mantle dynamics. However, analysis of experimental data on thermal transport properties of upper mantle materials highlights a large scatter of absolute values of thermal diffusivity. In particular, conduction of heat in single crystals is systematically higher than in polycrystalline samples. Here we present new thermal diffusivity measurements on San Carlos olivine single crystals and mantle rocks at 300K. Measured components of the olivine thermal diffusivity tensor along the <100>, <010>, and <001> directions are 2.73, 1.70, and 2.49 mm2. s-1, respectively. Measurements of thermal diffusivity of peridotites as a function of structural orientation show absolute values and anisotropy in good agreement with those predicted by petrophysical models based on the crystal preferred orientation of olivine and the above thermal diffusivity tensor. This suggests that the upper mantle thermal diffusivity is up to 50% higher than indicated by previous measurements on mantle rocks.