Pleurotus sajor-caju was cultivated on soybean straw for analyzing the nutritional content of spent mushroom straw (SMS). Results showed an increase in protein (3.60 to 5.85%), ash (7.50 to 9.20%) and moisture (7.00 to 8.30%) content while a decrease in pH (7.20 to 5.5o%), Crude fiber (41.50 to 22.00%), total carbohydrates (81.00 to 51.60%), lignin (29.00 to 12.20%), cellulose (39.10 to 28.50%), hemicellulose (21.30 to 08.50%) and tannin (38.80 to 23.33%) content in spent straw as compare to soybean straw. During vegetative growth of the fungus (spawn run period), lignin degradation was faster and during fructification, lignin degradation was slower while cellulose and hemicellulose degradation in straw was faster in rate during fruitification. The individual amino acid concentration in spent straw was also found to be increased. The cultivation of Pleurotus sajor-caju on soybean straw increased the nutritional content of spent straw. The protein content of spent straw increased while the lingo cellulosic content was reduced, making it more digestible and thus improved the potential feeding value of the resultant substrates as feed resources for ruminants.