This study was conducted to investigate the effect of post-harvest CO2 treatment on the quality of the ‘Gonji-7ho’ oyster mushroom. The harvested mushrooms were pre-cooled at 3oC for 1 day and placed in a gas-tight chamber with 0%, 30%, or 50% of CO2 concentration for 3 hours at 3oC. Next, 400 g of the oyster mushroom sample was packaged into 20-μm thick oriented polypropylene (OPP) film bags and stored at 4oC for 21 days. Treatment with 30% of CO2 treatment maintained the highest stipe firmness of the oyster mushrooms during storage. The stipe lightness (CIE L*) was the highest at 14 and 21 days, while the stipe yellowness (CIE b*) was the lowest at 2 and 7 days of storage. Therefore, we concluded that the 30% CO2 treatment maintained the overall visual quality of the ‘Gonji-7ho’ oyster mushroom until 17 days of storage at 3oC. Our results suggest that the shelf life of ‘Gonji-7ho’ oyster mushroom could be extended by the postharvest application of 30% CO2 for 3 hours during low temperature storage.