Abstract: The fermentation medium components for xylanase production by Trichoderma reesei was optimized by response surface methodology (RSM) . Firstly, three of the most significant influence factors were screened by the method of Plackett-Burman design as lactose, corn syrup and KH2PO4. The path of steepest ascent was applied to approach the optimal region of the three significant factors. Lastly, the optimal concentration and correlations among three factors were identified by RSM. The optimal fermentation conditions were determined as followed:lactose 45. 13g/L, corn syrup 15. 94g/L, (NH4) 2SO4 3g/L, KH2PO4 2. 73g/L, MgSO4·7H2O 0. 8g/L, CaCl20. 6g/L, Tuween-80 1mL/L. Under these conditons, the maximum theoretic activity of xylanase was 855. 01U/mL. After five parallel verifications, the maximum theoretic value was consistent with mean value of verification test and the xylanase production was increased by 24. 1% compare to that before optimization.