The study was conducted to determine the effects of chitosan on content of arachidonic acid and the activity of phospholipase A2 (PLA2) in serum, and relative expression of cytosolic phospholipase A2 (cPLA2) mRNA in duodenum, jejunum, and ileum of broilers. Two hundred and forty one-day-old Arbor Acre broilers were randomly allotted to six treatments with five replicates each and eight chickens in each replicate. Six groups were prepared for the study. Control group was the basal diet without chitosan. The other five groups were formulated by the addition of 50, 200, 500, 1 000 and 2 000 mg/kg of chitosan to the basal diet, respectively. The trial lasted for 42 days. The results showed that chitosan improved the content of AA and the activity of PLA2 in serum, and also the expression of cPLA2 mRNA in duodenum, jejunum, and ileum of broilers in a quadratic dosedependent manner (P<0.05), and the supplementation of 500 and 1 000 mg/kg chitosan to the diet were more effective. However, the supplementation of 2 000 mg/kg chitosan to diet was less effective. It was concluded that it was probably related to the improvement of PLA2 activity and the expression of cPLA2 mRNA in small intestine that chitosan may improve the immune function of broilers.