Abstract: Replacing several serine and threonine residues in the Ser/Thr surface of xylanase(XynB)with arginines effectively increased the thermostability of the enzyme by site-directed mutagenesis. The mutated enzymes XynB-104 and XynB-77 were expressed in Pichia pastoris and their enzymatic properties were determined. The optimal temperature of the mutations XynB-104 and XynB-77 were 50 ℃, and the relative activity of mutations were dramatically increased. Both mutated xylanases showed about 80% of maximal activity after incubating in xylan substrate for 2 h at 50 ℃ compared to only 26% activity of the native enzyme. The optimal pH of XynB-104 enzyme had no change, but the pH optimum of the XynB-77 enzyme was increased from 5.0 to 5.5. The results of the mutated enzymes indicated that properly arginines residues introduced into Ser/Thr surface of xylanase XynB might be effective to improve the enzyme thermostability and hydrolysis capacity.