Xylanase (E.C 220.127.116.11) is a widespread group of enzyme which can catalyze the endohydrolysis of 1, 4-β-D-xylosidic linkages in xylan. The thermostable xylanase has attracted widely attention because of its application in industrial processes. In this study, the cDNA of xylanase gene was obtained from Aspergillus niger C3486 by RT-PCR. The xylanase DNA contains a 50 bp intron through the comparison between the DNA and cDNA sequence. The thermostability of this xylanase was significantly improved by directed evolution.