3 Abstract: Six-hundred forty broilers were allotted into four dietary supplementation of none, 55 pp m bacitracin, 2.5 ppm nosiheptide, or 55 ppm Oxytetracycline (OTC) in a corn-soybean meal basal diet with eight pens in each treatment to investigate the effects of added various Antibiotic Growth Promoters (AGP) in diet on growth performance, immune response and gastrointestinal physiology. Data showed that nosiheptide supplementation but not bacitracin or OTC increased feed efficiency in starter period. The nosiheptide decreased the small intestine weight, IgA-positive cells along the intestinal tract and ileal mucosal secretory IgA (sIgA) concentration but bacitracin and OTC increased the IgA-positive cells in ileum. The nosiheptide supplementation group also elevated ileal sucrase and maltase activities and reduced the Lactobacillus counts in rectal contents as compared to the other treatment groups. In conclusion, the enhanced disaccharidase activity, diminished rectal Lactobacillus counts and reduced immune activated status are possibly the important factors for the improvement of feed efficiency of broilers on dietar y nosiheptide supplementation from days 0-21.