The present study was designed to evaluate the influence of different substrates on the yield and antioxidant properties of oyster mushroom (Pleurotus ostreatus (Jacq.) P. Kumm.) cultivated in grow bags. The substrates used in this study were: (1) maize straw, (2) beech sawdust supplemented with wheat bran at a rate of 20 %, (3) mixture of maize straw and spent coffee grounds in a ratio of 70: 30, and (4) mixture of maize straw and spent coffee grounds in a ratio of 50: 50. Total phenolics, total flavonoids and total antioxidant activity of oyster mushroom were determined by the Folin-Ciocalteu method, Aluminum chloride method and Ferric Reducing Antioxidant Power assay respectively. The highest fruiting bodies yield of oyster mushroom was obtained from substrate 4 while the least was from substrate 1. Total phenolic contents ranged from 3.80 mg in oyster mushroom grown on substrate 4 to 4.85 mg of gallic acid equivalents g-1 dry mass in oyster mushroom from substrate 2. Total flavonoid contents were very low in all analysed mushroom extracts. There was no significant difference between total antioxidant activities of oyster mushroom grown on different substrates.