Mechanisms and P-T dependence of thermal conductivity and diffusivity of minerals are reviewed. There are three heat transfer mechanisms: lattice, radiative and electric heat transfers. Lattice and radiative heat transfers are very important to the thermal regime in the mantle. Thermal diffusivity is an essential physical quantity to lattice heat transfer, whereas thermal conductivity is to radiative one. The mean free path of the particles carrying heat is the most important factor to discuss both thermal conductivity and diffusivity. In the case of lattice heat transfer, the mean free path is limited by the anharmonicity of lattice vibrations. Since the effect of the anharmonicity is approximately proportional to the absolute temperature, thermal diffusivity is inversely proportional to it. On the contrary, thermal diffusivity increases with increasing pressure, because the increase of pressure slightly decreases the effect of anharmonicity. The radiative heat transfer increases with increasing temperature according to the Stephan-Bolzmann's law. Because of the shortening of the mean free path of photon with increasing temperature, however, the radiative heat transfer increases in the smaller rate than T⁴. The pressure dependence of the radiative heat transfer may be negative.