This study investigated the effect of the difference in the cooling rates on the optical properties of zirconia during the simulated firing of porcelain, without a porcelain layer on a zirconia core ceramic. No difference was observed in the average transmittance of zirconia with the cooling rate during simulated firing (p>0.05). In all groups, the average transmittance decreased from approximately 44% to approximately 28% (p<0.001) as the thickness increased from 0.51 mm to 2.0 mm. No difference was observed in the translucency and opalescence parameters of zirconia with the cooling rate during the simulated firing of porcelain (p>0.05). In all groups, the translucency decreased from approximately 16 to approximately 5 (p<0.001), while the opalescence increased from approximately 6 to approximately 11 (p<0.001) as the thickness increased from 0.51 mm to 2.00 mm. Thus, the average transmittance and translucency parameter decreased exponentially as the thickness increased in all groups regardless of the cooling rate during simulated porcelain firing, while the opalescence parameter increased in a parabolic manner. Therefore, in this study, even if porcelain is fired at a cooling rate higher or lower than the typical cooling rate when manufacturing a prosthesis with a zirconia core, the optical properties of zirconia are not expected to be significantly affected.