Screening of isolates for acid protease production was car-ried out on enrichment media containing (w/v): yeast extract(0.5%), polypeptone (1%), soluble starch (2.0%) and potassium phosphate (0.27%). A potential strain was identified to produce high levels of protease. In addition, the parent strain was subjected to mutation by UV mutagenesis using an ultraviolet germicidal lamp on a clean bench (Sanyo, Japan). Briefly,conidia of A. oryzae RIB40 were spread into PDA plates and after UV treatment, were incubated at 30◦C for 3 or 4 day suntil the colonies appeared. Then any colonies showing morphology different from that of the parent strain were picked out, followed by inoculation and cultivation as a single colony on a PDA plate. The resulting strains were subjected to acid protease assay.