Abstract: Fermentation conditions of xyalnase production by Streptomyces sp. D21 were optimized in the study. The results of single-factor experiment showed that insoluble corncob xylan was the best carbon source and inducer, beef peptone was the best nitrogen source, the optimal initial pH value and cultivation temperature were 6.0 and 40℃, respectively. Under the optimal conditions, the highest xylanase activity of 1 128 U/mL was obtained after four days of cultivation.