Abstract: grass carps of 3.74?.23g were used in an experiment,the effect of small peptides on growth performance of grass carp Ctenopharyngodon idella.The fish was divided into three diet groups and reared in 3 triplicate aquarium(0.25m3) with 25 fishes in each aquarium for 56 days.The diets of test group 1,2 and control group included in 0.5% enzymatic casein(small peptides and average length of peptides 2.98),0.5% acidolysic casein(free amino acids)and 0.5%casein.The result showed that the relative growth rates(RGR),feed conversion ratio(FCR),protein efficiency ratio(PER),contents of magnesium and small peptides in blood plasma on fish in 0.5% enzymatic casein group were significantly higher than those in 0.5% casein group and 0.5% acidolysic casein group (p<0.05).These results indicated that small peptides could improve the ratio of apparent digestion and protein digestion and increase the total amount of small peptides in blood circulation and the retention of nitrogen in body,accordingly reducing the store of lipid in hepatopancreas and mesentery,and to advance the utilization of protein in diet.