A novel salt-tolerant acid protease was produced from Aspergillus oryzae LK-101 (AOLK-101). The AOLK-101 protease was purified to homogeneity by ammonium sulfate precipitation, DEAE-Sephadex A-50 and Sephadex G-100 chromatographies in order. The specific activity and the purification ratio of the purified protease were 2,301 unit/mg and 11.6 fold, respectively, with 25 kDa of molecular weight on sodium dodecyl sulfate-polyacrylamide gel electrpphoresis (SDS-PAGE). Its optimal pH and temperature were pH 6.5 and $50^{circ}C$, respectively. This protease was relatively stable at pH 4.5-7.5, below $40^{circ}C$, and up to 10% salt concentration. The protease was moderately inhibited by $Ag^{2+}$ and $Zn^{2+}$, and strongly by ethylenediamide tetraacetic acid (EDTA) and phenylmethysulfonyl fluoride (PMSF), but activated by $Cu^{2+}$ and $Mn^{2+}$. Therefore, the AOLK-101 protease was a serine protease based on the influence of metal ions and inhibitors. $K_m$, $V_{max}$, $k_{cat}$, and $k_{cat}/K_m$ values of AOLK-101 protease for hammastein milk casein were 1.04 mg/mL, 124.84 unit/L, 163.5/sec, and $3.9{ imes}10^6/m{cdot}sec$, respectively.