Metagenomics is a culture-independent method that allows the identification and characterization of organisms from all types of samples. So, this study aimed to use a metagenomics approach to compare the benefits of two commercially available growth promoters namely synbiotic (Micro-BACLA)^® and antibiotic (zinc bacitracin) ^® on microbial biodiversity and immunity properties in poultry. In total, 120-day-old Ross chicks from an Egyptian commercial hatchery were divided into three groups (n=40) with four repetitions per treatment. The groups namely (G1) control group, (G2) antibiotic group, (G3) synbiotic group. At the phylum level, five different phyla of bacteria were detected, Firmicutes had the largest representation with 93.87, 90.94 and 91.38% in G3, G2, and G1, respectively. At the family level, Lactobacillaceae and Bacillaceae were dominant in G3, while Lachnospiraceae represented the largest percentage in G1. Furthermore, Ruminococcaceae was the highest family in the antibiotic group. At the genus level, Lactobacillus represented the highest percentage in G2 and G3 accounting for 23.91 and 35.28%, respectively.  Contrarily, the Lachnoclostridium was more prevalent in the G1 at a rate of 29.95% compared to other groups. Regarding the immune response, the serum IgG and total IgE levels were equal at 50 ng/ml and 0.20, respectively, and did not change in response to synbiotic or antibiotic supplements in fodders. Whereas, IgA and IgM in chicken' serum were affected by the supplement of fodder with synbiotic compared to antibiotic and control groups, both of them were decreased. The current study concluded that adding synbiotics to the chicken broilers diet could increase nutrient digestion and growth performance by minimizing the negative effects of antibiotics and maximizing the synbiotic's advantages in the gut.