In order to obtain mutants with high lipase production, the Rhizomucor miehei with 10 U/mL enzyme activity was selected as the original strain, and was treated by combining microwave irradiation and nitrosoguanidine(NTG). The optimum treating conditions were got by primary screening. A mutant named MN 4 with higher activity of lipase was obtained as the original strain was treated by a power of microwave irradiation 500 W for 100 s and then a NTG treatment for 60 min. The activity of lipase of MN 4 was 20.50 U/mL, which was 105% higher than that of the original strain. Then an experiment for studying the genetic stability was taken by means of passaging 5 times and its results showed that MN4 had a better genetic stability. The Lipase-producing conditions of such mutant were investigated and the optimum conditions were summed up as follows: fermentation temperature was at 30°C, pH value was 7.0, and fermentation time was 4 d. Under these conditions, the produced Lipase had the highest activity of 22.90 U/mL, which was 129.0% higher than that of the original strain. The experimental results showed that microwave and NTG mutation agents possessed analogical mortal ratio trend. It positively related to the mutation agent's concentration microwave-NTG composite mutation presents some synergies. This could weaken repellence and saturation by single mutation agent. Microwave-NTG composite mutation could realize the complementary advantages and inerease mutant efficiency.
