Abstract: Molecular weight and antitumor effect in vitro of small black bean whey peptides were studied. Whey protein was hydrolyzed by papain, 537 acid proteinase and Prote AX protease separately. Results indicated that molecular weight distribution of Prote AX enzymolysis polypeptide was obviously different from the other two polypeptides. It indicated that H 22 tumor cell growth repression rate of Prote AX enzymolysis polypeptide was the highest, which reached to 51.36% when the peptide concentration was 40mg/mL. Mice were fed Prote AX enzymolysis polypeptide in AP-NaNO 2 -induced liver injury model. Results showed that MDA, ALT, AST and GSH-Px of mice in high dose group were significantly higher than those of mice in the model group (p<0.05) . Results of histology slice examination showed that liver cells in the model group were abnormal and injuerd seriously, pre-cancerous lesions were appeared. However, the liver cells of mice in high Prote AX enzymolysis polypeptide dose group were close to normal liver tissue. Therefore, the AP -NaNO 2 -induced liver injury and pre-cancerous lesions could be inhibited by Prote AX enzymolysis polypeptide.