The collagen peptide were prepared from scales of grass carp with pepsin,protamex,papain and alcalase.The effects of protease types and hydrolyzing conditions on the hydrolyzing degree,nitrogen recovery and gel strength of the hydrolysates were investigated.To obtain high gel strength collagen peptide,conditions for hydrolyzing were optimized with orthogonal experiment.The results showed that the nitrogen recovery of pepsin hydrolysate was much high,and the hydrolysate had the ability to form gel at low temperature.The results of orthogonal experiment indicated that hydrolyzing conditions had a significant effect on the hydrolyzing degree,nitrogen recovery and gel strength.To obtain higher gel strength and nitrogen recovery,the optimum value for substrate concentration,pepsin dosage,initial pH value and hydrolyzing temperature were 5.0%,4.0,140.0 U/g,and 65 ℃,respectively.The hydrolyzing degree of scales of grass carp by pepsin was 1.46%,with nitrogen recovery of 64.38%,gel strength of 2 051 g when hydrolyzing 120 minutes.