The volume and grain boundary diffusion of O-18 at 1673 K and magnesium and calcium distribution in polycrystalline alumina doped with MgO were studied using a resistive anode encoder of secondary ion mass spectrometry (SIMS). The second phase contains magnesium and calcium near the surface. The formation and growth rate on the second phase depends on the grain boundary orientation. The oxygen profile along grain boundary is influenced with the distribution of second phase, and this effect is important near the surface.