The study was conducted to investigate the effect of shading treatment on the uniform primordium formation and development of fruit body during incubation of major cultivated oyster mushrooms (Pleurotus ostreatus). Oyster mushrooms cultivars Suhan No. 1 and Gonji No. 7 were incubated while shading during different times in the incubation room wherein temperature of 20 oC was maintained. Substrate temperature during incubation was increased to 24~25 oC in 11~12 days for Suhan No. 1 and to 25~26 oC in 11~12 days for Gonji No. 7 regardless of shading times. The CO₂ generation was the highest as 9~11% for Suhan No. 1 and 9~10% for Gonji No. 7 at the time of temperature increase the highest, while O₂ contents became decreased, thus constructed good environment for the growth of hyphae. Therefore, ratio of ununiformal primordia formation was reduced by 20% as against control for Suhan No. 1 and by 13% for Gonji No. 7 when shading was applied from 10 th day of inoculation with no difference in fruit body yield.