Several strains of A. oryzae (3042,AF92011,AF930`8,AF93020,AF93022 and AF93332) were mutated by 1 methy 3 nitro 1 nitrosoguanidine. High producers of aminoacylase (3042 5, AF93020 2,AF93020 6和AF93022 5) were selected from these mutants. The Aminoacylase activities for resolving DL methioninewere were 34.1%, 157.6%,152.4% and 145.4% higher than that produced by wild type strains, respectively. Enzyme activity of the mutant A. oryzae 3042 5 from A. oryzae 3042 could reach to 203.1U/ml.