Magnesia powders with different particle sizes were prepared by a vapor phase oxidation process, and the effects of particle size and particle shape on their sinterabilities were investigated from a viewpoint of that densification progressed with simultaneous grain growth. For the magnesia powders with particle sizes such as 0.014 and 0.054μm, the densification began at lower temperatures, but the density of sintered bodies slightly increased at temperature above 1400°C. The rate of grain growth was higher than that of densification, because grains in sintered bodies constituted small radius of curvature of grain boundaries. On the other hand, for magnesia powders with particle sizes such as 0.126 and 0.199μm, sinterability was inferior to the small-sized powders at lower temperature, but the density of sintered bodies considerably increased up to 1650°C. The rate of grain growth was lower than that of densification, because most grains in sintered bodies were cubic. Consequently, it was found that grain growth as an integral part of controlling sintering phenomena was affected by the shape of magnesia particles.