Background:Effective antibiotic alternatives are urgently needed in the poultry industry to control disease outbreaks. Phage therapy mainly utilizes lytic phages to kill their respective bacterial hosts and can be an attractive solution to combating the emergence of antibiotic resistance in livestock. Methods:Five hundred and four, one-day-old broilers (Ross 308) were allotted to 1 of 4 treatment groups in a completely randomized design. Treatments consisted of CON (basal diet), PC (CON+0.025％Avilamax?), BP 0.05 (CON+0.05％bacteriophage), and BP 0.10 (CON+0.10％bacteriophage). Results:A significant linear effect on body weight gain (BWG) was observed during days 1–7, days 22–35, and cumulatively in bacteriophage (BP) supplemented groups. The BWG tended to be higher (P=0.08) and the feed intake (FI) was increased (P=0.017) in the PC group over CON group. A greater (P=0.016) BWG and trends in increased FI (P=0.06) were observed in the experiment in birds fed PC than CON diet. At the genus level, the relative abundance of Lactobacillus was decreased in PC (65.28％), while it was similar in BP 0.05 and BP 0.10 (90.65％, 86.72％) compared to CON (90.19％). At the species level, the relative abundance of Lactobacillus salivarus was higher in BP 0.05 (40.15％) and BP 0.10 (38.58％) compared to the CON (20.04％) and PC (18.05％). A linear reduction in the weight of bursa of Fabricius (P=0.022) and spleen (P=0.052) was observed in birds fed graded level of BP and an increase (P=0.059) in the weight of gizzard was observed in birds fed PC over BP diets. Linear and quadratic responses were observed in redness of breast muscle color in birds fed graded level of BP. Conclusions:The inclusion of the 0.05％and 0.1％BP cocktail linearly improved broiler weight during the first 7 days, 22–35 days and cumulatively, whereas 0.05％BP addition was sufficient for supporting immune organs, bursa and spleen as well as enhancing gut microbiome, indicating the efficacy of 0.05％BP as a substitute antibiotic growth promoter in broiler diets.