Casein hydrolysates were prepared by hydrolysis of casein with papain, and then modified by papaincatalyzed plastein reaction to prepare antioxidant peptides with higher activity. Response surface analysis was conducted to optimize the co nditions of plastein reaction with the variations of free amino groups in reacti on mixture used as index. The optimal conditions obtained were that enzyme addit ion level was 500U〖DK〗/g, temperature was 30℃, concentration of hydroly sates was 50% and reaction time was 56h. The analysis results from capillary electrophoresis confirmed that the peptide compositions of modified products we re different from that of casein hydrolysates. The antioxidant activities of thr ee modified products were also analyzed, which indicated that their scavenging a ctivities for three free radicals were enhanced obviously.