Abstract: The vitro antioxidant activities of tilapia hydrolysate and its 5 ku ultrafiltrate were studied aimed at offering basic theory for tilapia processing and high-value utilization. The capacities on reducing power，·OH，O2 -·， DPPH·free radical scavenging and its antioxidation in linoleic acid of hydrolyzate and ultrafiltrate were investigated by spectrophotometry，as well as an in-depth analysis on its antioxidant activity variability was carried out. The results showed that when the concentration was 1.25-2.5 mg/mL，the reducing power and antioxidation in linoleic acid of ultrafiltrate was lower than those of hydrolyzate，while the concentration was 2.5-10 mg/mL，the result was opposite. And the free radical scavenging of ultrafiltrate was better than that of hydrolyzate with half inhibitory concentration （IC50）. The contents of Leu，Phe，Ile，His and Tyr in ultrafiltrate were higher than those in hydrolysate，in addition， the components which were lower than 3 ku or 1 ku，in ultrafiltrate were higher than those in hydrolysate. The results indicated that compared with tilapia hydrolysate，the antioxidant activity of 5 ku ultrafiltrate obviously increased