Abstract In this study, we investigated the effect of the average grain size on the transmittance and mechanical strength of high purity sintered alumina. When the grain size was reduced from 2 to 0.8 μm, the transmittance abruptly increased due to Rayleigh scattering. However, sintered alumina with a grain size of between 2 and 7 μm showed no transmittance. The transmittance slightly increased when the grain size was increased to more than 7 μm. The bending strength of the sintered alumina increased with decreasing grain size, which was well matched with the theoretical considerations. Polycrystalline alumina with an average grain size of 0.8 μm exhibited high transmittance and high mechanical strength.