The growth property of Streptomyces sp.,a xylanase producing strain,isolated from the yak dung in Qinghai-Tibet plateau and the nature of xylanase were studied in this paper.Growing in fermentative medium,the strain can produce xylanase(3227.346U/mL).We delt with the original strain by the heavy-ion irradiation.We screened a high-yield xylanase producing strain SZ10-7 from many of mutants and its enzyme activity reached to 5 338.42 U/mL.Compared with the original strain,the enzyme activity of mutant strain SZ10-7 increased by 1.65 times.The fermentation conditions of the mutant strain SZ10-7 were optimized and its enzyme activity further increased,reaching to 5850.20 U/mL.The mutant's optimum conditions for enzyme production consisted of 5% the mixture of the corn cobs and the wheat bran(1∶ 1),0.8% yeast extract,0.1% K2HPO4·3H2O,0.5% MgSO4·7H20,0.3% NaCl,the initial pH 7.0,shaking culture for 96h,at 25℃.These results showed that the heavy-ion irradiation is an effective method for microbe mutagenesis.