To determine the optimum supplemental level of Entermorpha prolifera for broiler diet, this experiment was conducted to investigate the effects of different supplemental levels of Entermorpha prolifera on performance, carcass quality and small intestinal digestive enzyme activities of broilers. Four hundred 1-day-old healthy Arbor Acres broilers were randomly allocated to 4 groups with 5 replicates per group and 20 chickens per replicate. Control group was fed a basal diet, and test groups were fed the basal diet supplemented with 2%, 3% and 4% dry powder of Entermorpha prolifera, respectively. The experiment lasted for 35 days. The results showed as follows: 1) the average daily gain of broilers fed diets with 3% and 4% Entermorpha prolifera dry powder were significantly higher than that in the control group (P<0.05), the feed consumption of broilers fed diets with 3% Entermorpha prolifera dry powder was significantly higher than that in the control group (P<0.05), the ratio of feed to gain in all test groups was lower than that in the control group to some extent, while there were no significant differences among them (P>0.05). 2) Adding 2% to 4% Entermorpha prolifera dry powder in diets significantly decreased the thickness of subcutaneous fat, abdominal fat rate and the content of fat in breast muscle (P<0.05 or P<0.01), and adding 3% and 4% Entermorpha prolifera dry powder in diets significantly decreased the width of intramuscular fat (P<0.05). 3) The amylase activities in duodenal content of broilers were increased by adding 2% to 4% Entermorpha prolifera dry powder in diets to some extent, which only in 4% group was significantly increased (P<0.05). It is concluded that adding 2% to 4% Entermorpha prolifera dry powder in diets of broilers can improve the growth of broilers, decrease abdominal and subcutaneous fat, improve breast meat quality, and enhance the activities of amylase in duodenal content of broilers. The effects of 3% and 4% Entermorpha prolifera dry powder supplementation are better.