Highly transparent 8 mol% Y2O3-ZrO2 (8Y) ceramics were fabricated by the hot isostatic pressing method. The transmission in a visible light region was comparable to that of a single crystal. In this work, the microstructural relationship between presintered and hot isostatic-pressed material was examined. The transmission is sensitive to the microstructure of presintered material. The microstructural features of fine grains and small intergranular pores are significant to give high transparency for a resultant hot isostatic-pressed material. A theoretical model based on Mie scattering revealed that the residual pore with submicrometer size is responsible for the transmission in the visible light region. The porosity <100 ppm is required for high transparency.