Abstract: The contents in the rumen of fibrous matters, enzymatic activity, and nutrient degradability of corn stalk treated by Pleurotus ostreatus was studied at 16, 20, and 24℃ to find the optimal range of treatment temperature and time to achieve a greatest nutritional value of corn stalk. P. ostreatus fermentation reduced the contents of cellulose, hemicellulose, and lignin in stalks treated at the three temperatures. Following treatment of corn straw with P. ostreatus, fibrous matter was degraded slower (P<0.05) and the activity of fibrolysis enzymes (P<0.01) and dry matter degradability were reduced (P<0.05) in the rumen at 16℃ compared with treatment at 20 and 24℃ but were not significantly different between the temperatures. Dry matter degradability in the rumen of stalks treated at 20 and 24℃ increased with treatment time until 21 days of fermentation, and then remained constant. The optimal conditions to treat corn straw with P. ostreatus to achieve a greater nutritional value for ruminant animals is between 20 to 24℃ for 21 to 30 days or for 30 days at a minimum of 16℃.