The present study was performed in order to examine the effect of using marjoram extract as a natural growth promoter on performance and intestinal bacteria of broiler chickens. Marjoram extract was prepared by the maceration method. Then, an in vitro trial was conducted to determine the antibacterial activity of marjoram extract against six bacterial strains. The results of the antibacterial disk diffusion test showed that marjoram extract has a high inhibitory effect against both Gram-positive and Gram-negative bacteria. Later on, an in vivo feeding trial was done using 90 day-old Ross broiler chicks. The chicks were divided into two groups of three replicates each. The first group served as a control without any supplementation while the second group was supplemented with marjoram extract as a growth promoter. The results showed that addition of marjoram extract led to a significant (p<0.05) improvements in final live body weight, total weight gain and total feed intake of broiler chickens. On the other hand, the relative weights of internal organs were not affected by marjoram extract supplementation. The total aerobic intestinal bacteria count of the broilers fed the marjoram extract supplemented diet was significantly lower (p<0.01) than that of the control group. The overall results suggest that marjoram extract could successfully be used as a natural growth promoter alternative to antibiotic in broilers' diets.