Because of secreting large amounts of enzymes, Aspergillus oryzae was widely used in the fermentation of soy sauce in many Asian countries. Here, N+ ion implantation mutagenesis was applied to improve the ability of A. oryzae to secrete acid protease. Several mutants were screened using three kinds of plates (Casein medium, Czapek's medium and carboxymethyl cellulose medium agar plates). Compared with the other mutants, the mutant A100-8 could secrete the most protease. Acid protease activity of A100-8 was enhanced about 44.1% at 36 h by koji fermentation test. In addition, A100-8 was stable by periodically subculturing and maintaining on the agar slant. The mRNA expression levels of two kinds of acid protease (serine carboxypeptidase and aspartyl protease) were measured by RT-PCR at different periods. Serine carboxypeptidase and some kinds of aspartyl protease of A100-8 were significantly (P < 0.01) expressed higher than the control at all times.