Study on the nutrient quality assessment of Pleurotus ostreatus grown on agrowastes composted for five months was carried out in Department of Plant Science and biotechnology and Dilomat Farms and Services Limited all in Rivers State University. Substrate materials such as sawdust, Rhizophora wood ash and Rice bran were composted for five months to grow Pleurotus ostreatus. The Association of Analytical Chemists (AOAC) method was adapted for the determination of antinutrient and nutrient components. Proximate evaluation revealed the presence of moisture (33.36 + 0.05) ash (0.96 + 0.04) lipid, fiber, carbohydrates, protein. Mineral and vitamin investigation showed high values of calcium (276.67 + 1.23) iron, magnesium, phosphorus, potassium, sodium and thiamine (0.77 + 0.06). Anti-nutrient investigation revealed the availability of glycoside, oxalate, saponin, tannin, polyphenol, flavonoid and lignant. Generally, P. Ostreatus grown on five months composted soil had improved nutrient quality.