In order to study the effects of adding levels of Enteromorpha prolifera into a diet on immunity and serum biochemical indices of broiler chickens, two hundred and forty healthy one-day-old Avian broilers were randomly divided into four groups with six replicates in each group and ten chickens in each replicate. Enteromorpha prolifera levels in the diets of four groups were 0 (control group), 2%, 3% and 4%, respectively. The experiment lasted for 42 days. The results showed that, compared with the control, 3% adding level could significantly increase the antibody titer of Newcastle disease after secondary immunization, total protein and albumin contents in serum (P<0.05); and 4% adding level could significantly increase the transformation rate of peripheral blood T lymphocyte at the later stage, immune organ indices, high density lipoprotein cholesterol and albumin contents in serum, decrease total cholesterol, triglyceride and malonaldehyde contents in serum, enhance total superoxide dismutase and glutathione peroxidase activities in serum (P<0.05 or P<0.01); while each adding level had no significant difference on the glutamic pyruvic transaminase, glutamic oxalacetic transaminase, urea nitrogen and globulin contents in serum (P>0.05). In conclusion, Enteromorpha prolifera, a chlorella that has no toxicity, could enhance the immunity and antioxidant function, improve lipid and protein metabolism of broiler chickens, and it could be used with a level of 3%～4% in production.［Chinese Journal of Animal Nutrition, 2010，22（3）:682-688］