Thermal expansion of MgO, periclase, is approximated by contributions from a quasi-harmonic perfect crystal and its equilibrium high temperature/pressure defects. We demonstrate that the influence of thermal defects on high pressure thermal expansion is very small and may be neglected at lower mantle conditions. Following that, a simple relationship has been obtained between high pressure thermal expansion and the thermal expansion of a quasi-harmonic perfect crystal at zero pressure. Our estimation of MgO thermal expansion at the core-mantle boundary is 9.36¿10-6K-1. ¿ American Geophysical Union 1995