The effect of three different levels of mannan oligosaccharides (MOS) as a dietary supplement on carcass characteristics, meat quality and intestinal microbial ecology of growing Japanese quail (Coturnix japonica) was the main objective of the present experiment. A total of one hundred 1-day-old Japanese quails were randomly divided into 4 experimental groups (25 birds/ each treatment) with 3 replicates (8 birds in two replicates and 9 birds in one replicate) in each group. The birds of experimental groups were fed on four dietary treatments: 1) a basal diet without supplementation (control); 2) a basal diet with 1 g MOS/kg diet (low MOS); 3) a diet with 3 g MOS/kg diet (medium MOS); and 4) a diet with 5 g MOS/kg diet (high MOS). The experimental period extended for 42 days. The data revealed that, birds fed diets containing medium MOS level (3 g /kg feed) recorded significant (P < 0.05) improvements in body weight and weight gain compared with other treatment groups. Medium level of MOS supplementation increased the dressing and edible giblets percentages, while the offal's and carcass abdominal fat percentages were significantly decreased. Crude protein and moisture values of quail’s meat were higher in medium MOS supplemented birds diet than in other groups, while fat and ash values were lower. Total aerobes and E. coli counts were not significantly differed (P>0.05) between the treated groups. A significant increase in lactobacilli counts were detected in duodenum and jejunum of MOS supplemented groups diet. Birds fed medium MOs supplemented diet showed the highest lactobacillus counts. From the results of the current study, it could be concluded that medium level (3g MOS/Kg feed) of mannan oligosaccharide improve the carcass characteristics, meat quality and intestinal microbial ecology of growing Japanese quails by increasing the growth of beneficial microbes and reduction of potential pathogens.