A 3×3 factorial experiment was performed to study the effect of supplementing broiler diets with different levels of Mannan-oligosaccharides (MOS) (0, 4 and 8 g/kg diet), Enterococcus faecium (E.f) (0, 1×10⁹ and 2×10⁹ CFU/kg diet) and their combination on growth performance and carcass traits of broiler chickens. A total number of 360 1-week-old broilers were randomly distributed to 9 equal groups, each group contained 40 unsexed birds with 4 replications (10 birds each). Results showed that average live body weight (LBW), body weight gain (BWG) and feed conversion ratio (FCR) of broiler chicks received diets containing 4 and 8 g MOS/kg were better than control group. While feed intake (FI) was not significantly affected by different levels of dietary MOS. The addition of 1×10⁹ CFU of E.f to the basal diet of broiler chicks insignificantly enhanced the increase of LBW, BWG and FCR, while the addition of 2×10⁹ CFU of E.f significantly increased LBW, BWG and FCR, as compared to control. No significant differences were found between groups treated with tested probiotic in LBW, BWG and FCR. The amount of feed consumed through the experiment was not significantly affected with E.f supplementation at different levels. No significant effects were detected due to dietary treatment with MOS, E.for their interaction on all studied carcass traits (carcass, liver, gizzard, heart, total giblets and dressing percentages). The results indicated that supplementing broiler diets with different levels of MOS, E.fand their combination has positive effect on growth performance with no significant effect on carcass traits.