Cellular damage caused by reactive oxygen species (ROS) has been implicated in several diseases, and hence antioxidants have significant importance in human health. In this study, five methods were used to test and compare the antioxidant activity at different levels involving formation and scavenging of free radicals by the extracts of four mushrooms that have medicinal values. The results were expressed as Trolox equivalent antioxidant capacity (TEAC) and ascorbic acid equivalent antioxidant capacity (AEAC). In the DPPH (1,1-diphenyl 2-picryl hydrazyl) assay, the ethyl acetate extract of Phellinus rimosus showed more potent activity than the methanolic extracts of Pleurotus florida, Pleurotus sajour-caju, and Ganoderma lucidum. The ethyl acetate extract of P. rimosus at a concentration of 0.1% showed a high TEAC value (12.488). In the ABTS (2,2-azobis-3-ethylbenzthiazoline-6-sulfonic acid) spectrophotometric assay, it possessed the most effective antioxidant activity (TEAC 4.84) compared to methanolic extracts of P. florida, P. sajour-caju, and G. lucidum. The extract of P. rimosus also possessed higher activity compared to other extracts in the ferric-reducing antioxidant power (FRAP) assay. In the pulse radiolysis studies and the ORAC (oxygen radical absorbance capacity) assay also, these mushrooms showed significant activities. Results of the DPPH, ABTS, FRAP, and ORAC assays indicate that all the four mushrooms examined showed significant antioxidant activities. Among these, P. rimosus extract seems to be the more effective antioxidant.