Pleurotus eryngii, known as king oyster mushroom, has been widely used in nutritional and medicinal purposes. This study was initiated to screen the antioxidant activities, tyrosinase inhibitory effects on the fruiting bodies of P. eryngii extracted with acetone, methanol and hot water. The antioxidant activities were performed on β-carotene-linoleic acid, reducing power, 1,1-diphenyl- 2-picrylhydrazyl free radical scavenging, and ferrous chelating abilities. In addition to this, phenolic acid and flavonoids contents were also analyzed. Methanolic and acetonic extracts of P. eryngii showed the strongest β-carotene-linoleic acid inhibition as compare to hot water extract. At 8 mg/ml, acetonic extract showed a high reducing power of 1.203. The scavenging effects on 1,1-diphenyl-2-picrylhydrazyl radicals, acetonic extract was effective than other extracts. The strongest chelating effect (88.10%) was obtained from the methanolic extract at 1.0 mg/ml concentration. Antioxidant activities of the extracts from the fruiting bodies of P. eryngii were increased with the increasing concentration. After application of reverse phase high performance liquid chromatography, coupled to a diode array detector and electrospray ionisation mass spectra, ten phenolic compounds namely, gallic acid, protocatechuic acid, chlorogenic acid, vanillin, ferulic acid, naringin, naringenin, hesperetin, formononetin and biochanin were identified from acetonic extract. Tyrosinase inhibition of acetonic, methanolic, and hot water extracts of P. eryngii were increased with the increasing of concentration. Results revealed that acetonic and methanolic extracts showed good, while hot water showed moderate activities of the tyrosinase inhibition at the concentration tested. This study suggests that fruiting bodies of P. eryngii can potentially be used as a readily accessible source of natural antioxidants.