We evaluated the antioxidant activity and tyrosinase inhibitory effects of Pleurotus ostreatus fruiting bodies extracted with acetone, methanol, and hot water. The antioxidant activities were tested against $eta$-carotene-linoleic acid, reducing power, 1,1-diphenyl-2-picrylhydrazyl free radical scavenging activity, and ferrous chelating ability. Furthermore, phenolic acid and flavonoid contents were also analyzed. The methanol extract showed the strongest $eta$-carotene-linoleic acid inhibition as compared to the other exracts. The acetone extract (8 mg/mL) showed a significantly high reducing power of 1.54 than the other extracts. The acetone extract was more effective than other extracts for scavenging on 1,1-diphenyl-2-picrylhydrazyl radicals. The strongest chelating effect (85.66%) was obtained from the acetone extract at 1.0 mg/mL. The antioxidant activities of the extracts from the P. ostreatus fruiting bodies increased with increasing concentration. A high performance liquid chromatography analysis detected seven phenolic compounds, including gallic acid, protocatechuic acid, chlorogenic acid, naringenin, hesperetin, formononetin, and biochanin-A in an acetonitrile and 0.1 N hydrochloric acid (5 : 1) solvent extract. The total phenolic compound concentration was $188{mu}g$/g. Tyrosinase inhibition of the acetone, methanol, and hot water P. ostreatus extracts increased with increasing concentration. The results revealed that the methanol extract had good tyrosinase inhibitory ability, whereas the acetone and hot water extracts showed moderate activity at the concentrations tested. The results suggested that P. ostreatus may have potential as a natural antioxidant.