Abstract: The effects of initial pH value of substrate controlled by acid and by buffer solution on the yield of enzymatic hydrolysis were studied when pure xylan and alkaline extractives were hydrolyzed selectively by xylanase for preparation of Xylo oligosaccharides.The experiments indicated the xylanase showed optimal activity at pH?5.0. At the same conditions for enzymatic hydrolysis, both pure xylan and alkaline extractives, the yield of oligosaccharide from the substrate hydrolyzed at acid controlled pH was lower than that at buffer solution controlled pH, but it was more obvious to alkaline extractives. Moreover,when the substrate was alkaline extractives, the transparence of supernatant liquid of enzymatic hydrolysate was opaque. The results of enzymatic hydrolysis showed that the optimal pH value controlled by acid for pure xylan and alkaline extractive was 4.8 and 6.0 respectively.