This experiment is conducted to study the effects of dietary antimicrobial peptides (ABPs)—cecropin on small intestinal mucosal morphology and the number of immune active cells in Hy-Line Brown young roosters. Three hundred and thirty-six young roosters were selected, and randomly divided into seven treatments with four replicates in each treatment and twelve chicks in each replicate. The seven treatments were control group (fed a basal diet), antibiotics group (fed basal diet+150 mg/kg chlortetracycline), and 150, 200, 250, 300, 350 mg/kg ABPs groups (fed basal diet+cecropin), respectively. The feeding experiment lasted for 42 days. The results showed as follows: 1) the epithelial mucous membrane structure of duodenum, jejunum and ileum in ABPs group were integrated compared with that in control group. Except that there were no significant differences in crypt depth of duodenum and muscularis thickness of jejunum among all treatments (P>0.05), villus height, mucosal thickness and muscularis thickness in ABPs groups were higher than those in control group in a certain extent, crypt depth in ABPs groups was lower than that in control group in a certain extent, and ABPs group Ⅴ had the best effect, and was significantly higher than control group about above those indices (P<0.05). 2) The number of intraepithelial lymphocytes of jejunum in ABPs group Ⅴ was significantly higher than that in control group (P<0.05). 3) The number of goblet cells of duodenum, jejunum and ileum in ABPs groups Ⅲ, Ⅳ and Ⅴ was significantly increased compared with that in control group (P<0.05). In conclusion, adding different levels of cecropin to the diet can improve mucosal morphology of small intestine, and increase the number of immune active cells. Therefore, the optimal additive amount of cecropin is 350 mg/kg.