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 (). Oyster mushrooms cultivars Suhan No. 1 and Gonji No. 7 were incubated while shading during different times in the incubation room wherein temperature of 20ºC was maintained. Substrate temperature during incubation was increased to 24~25ºC in 11~12 days for Suhan No. 1 and to 25~26ºC in 11~12 days for Gonji No. 7 regardless of shading times. The CO 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 O 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 day of inoculation with no difference in fruit body yield.