Round scad (Decapterus maruadsi),a kind of marine fish,belongs to the family of mackerel.It is usually regarded as a byproduct in fishery,discarded or processed into fish meal or fertilizer.The application of enzymatic hydrolysis under controlled conditions can yield desirable functional properties and pave the way for full utilization of this species.In this study,round scad muscle protein hydrolysates with different degrees of hydrolysis (DH) were obtained with alcalase.Three in vitro methods of DPPH? scavenging capability,O2?? scavenging activity,and reducing power were used to evaluate the antioxidative activities of hydrolysates.The antioxidative activity and amount of antioxidative amino acid for hydrolysates increased with the increasing of DH,and showed the maximum antioxidative ability when DH reached to 23%.An increase of peptides with molecular weight below 1500 Da was observed as the DH increased for round scad protein hydrolysates.It is revealed that DH greatly influenced the molecular weight and free amino acid composition,and further affected the antioxidative activity.Moreover,Box-Benhnken design was employed to investigate the influencing factors on DH for round scad protein and a quadratic regression model was obtained.The results indicated that all of the initial substrate concentration,initial enzyme concentration,pH and temperature exhibited significant.