Acid protease produced from Aspergillus tubingensis was pruified by ethanol fractionation, dialysis, and DEAE cellulose column chromatography. As a result of purification its specific activity increased to 5.4 times, and percent recovery was 39. The kinetic constants of the enzyme were studied. Km and Vmax was $1.5{ imes}10^{-7}M;and;0.11{Delta}O.D/min$ , respectively, when casein was used as substrate. The order of Km value of several proteins is : casein<hemolobin<BSA<cytochrome C, and that of Vmax is : myoglobin<casein<hemoglobin<BSA<cytochrome C. In case of BSA, substrate inhibition pattern was found. The enzyme was remarkably inhibited by EPNP, TPCK and NBS which inhibit carboxyl and tryptophan groups in the active site, and this property is similar with that of pepsin. The results of inhibition study show that carboxyl group of aspartic acid or glutamic acid takes roll in neucleophilic attack to peptide bond, and tryptophan group is involved in the binding site.