The effect of dietary supplement of arginine on immunoglobulin, cytokine and nitric oxide (NO) synthesis in broilers was investigated. Three hundred one-day-old Arbor Acre mixed-sex broiler chickens were randomly allotted to five dietary treatments with six replicates in each treatment and ten chickens within each replicate. The five treatments viz., the basal diet with 0 (arginine-deficient group), 4.5 (arginine-normal group, control), 9.0, 13.5,and 18.0 g/kg arginine. The feeding experiment period was 42 days. The results of in vivoexperiments showed that with increasing levels of dietary arginine, interleukin (IL)-2, NO content and inducible nitric oxide synthase (iNOS) activity in serum as well as iNOS mRNA expression in the ileum and liver of broiler chickens increasedquadratically at the age of 21 d(P<0.05).In addition, positive effects of arginine tended to be suppressed when addition of arginine in the diet was increased to 18.0 g/kg. The results of in vitro experiment indicated that, IL-2 and iNOS content increased quadratically with increasing addition of arginine (P<0.01), and 80 μg/mL arginine supplement group had the highest value. Dietary arginine improved the levels of IL-2, NO and iNOS expression in a quadratic dose-dependent manner.