Abstract: Experiments were conducted to examine the protein degradation during the extensive hydrolysis of Decapterus maruadsi by Flavourzyme, using nitrogen recovery, degree of hydrolysis (DH), TCA insoluble nitrogen and gel filtration chromatography. Results showed that most of TCA insoluble protein was soluble in water after 6 h of hydrolysis, with 83.26% nitrogen recovery and 41.8% DH. Prolonging the hydrolysis reaction after 6 h did not result in obvious increase in the nitrogen recovery, due to the decrease in the substrates available. The DH increased progressively from 41.8% to 59.7% until 21 hours. Prolonging the reaction after 21 h resulted only in a minimal increase in the DH. The main substrates for proteolytic hydrolysis after 21 h of extensive hydrolysis were identified as peptides with molecular weight range from 6214 to 10700 D.