A strain XC-1 with high milk-clotting activity was isolated from the merchant cheese.Using the physiological and biochemical identification and 16S rDNA sequencing,XC-1 was identified as Bacillus subtilis.It was the first time that N+ Ion beam implantation was applied for mutagenesis of Bacillus subtilis.Using XC-1 as parent strain,the mutation experiment showed that the dose of 2.08×1015 ions/cm2 with the power of 20 keV was optimal for mutagenesis of Bacillus subtilis and a mutant strain XCYB-6 was screened.The milk-clotting activity of XCYB-6 reached 2181.82 SU/mL and was 65.63% higher than that of the parent strain.Meanwhile the proteolytic activity of XCYB-6 was 2.53 U/mL.The ratio of milk-clotting activity to proteolytic activity reached 862.38.The pass-generation(six generations) test indicated that XCYB-6 had good genetic stability.