Highly sinterable powders are required for the fabrication of transparent ceramics. Here, we studied the effects of calcination atmosphere on the characteristics of monodispersed spherical Y2O3 powders, such as crystallite size and particle density, for high optical transparent ceramics. It was found that vacuum calcination around the crystallization temperature is the crucial step to eliminate intragranular pores in the spherical particle. The fast decomposition rate in a vacuum creates smaller crystallites, and the following higher calcination temperature results in the enhancement of pore elimination. The in-line transmittance of the transparent Y2O3 ceramics, vacuum sintered at 1750 degrees C, was improved by increasing the particle density of the as-calcined powders. This result indicates that the high-density starting particles effectively enhance the pore elimination during the fabrication of transparent Y2O3 ceramics.