The effect of three levels of mannan oligosaccharides (MOS) as a dietarysupplement on growth performance, carcass characteristics and some blood metabolites of growing Japanese quails (Coturnix japonica) was the main objective of this 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). The experimental groups consisted of four dietary treatments: 1) a control basal diet without supplementation; 2) a basal diet with a MOS at level of 1 g/kg feed (low MOS); 3) a diet with MOS at a level of 3 g/kg feed (medium MOS); and 4) a diet with a MOS at a level of 5 g/kg feed (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, weight gain, feed conversion efficiency, performance index, energy and protein efficiency compared with those fed the control one, low and high levels of MOS. Also medium level of MOS supplementation increased the dressing percentage and edible giblets, while the offal's percentages and carcass abdominal fat were significantly decreased. Growth performance was significantly improved at 3 g MOS/ kg diet, however high dosage of MOS have negative effects on growth rate of the birds. The triglyceride, total cholesterol and total lipids concentrations in quails blood decreased significantly in concomitant with a significant decrease in carcass fat % as a response to dietary MOS supplementation. From the results of the current study, it could be concluded that the responses of Japanese quails to MOS are influenced by the level of its supplementation in the diet. Therefore, this factor needs to be carefully considered in order to obtain maximal growth-promoting effects of MOS in Japanese quail production.