Streptomyces sp. X-435 isolated from a soil sample collected in the suburbs of Beijing was proved to be a produce Virginiamycin. To improve the productivity of the Virginiamycin of Streptomyces sp. X-435,the spores of strain X-435 were treated with UV. The three types of colony,strawhat, wrinkled, blad, were isolated on Gaose's medium plates after mutation. Among them, the colonies of strawhat type exhibited positive mutation and were picked up as objects of screening. After five generation of mutation, the mutant F5-25-u-28 was selected which potency of Virginiamycin was about 20times higher than that of the beginning strain by flask fermentation and was also genetic stable.