This study was conducted to investigate the effect of mannan-oligosaccharides (MOS) as nutritional supplementation on zoo-technical parameters, visceral organ weights, serum mineral levels and caecal concentrations of E. coli and Clostridium spp. in Japanese quail (Coturnix coturnix japonica). Japanese quail (one-day old; n = 1320) were taken and randomly separated into four groups (n = 320), having 8 replicates (n = 40) in each group. Corn-based poultry feed (basal diet) was given to control group birds (Group A) and the same feed mixed with three different concentrations of MOS i.e., 0.1, 0.5 and 1.0 %, was offered for 35 days to the birds of experimental groups, B, C and D, respectively. Results showed that the birds of group C were heavier (P < 0.05) during 2nd and 3rd weeks compared to the other groups. Feed conversion ratio was not affected by the dietary supplementation except on week 2, which was more (P < 0.05) in the group B liken with the group A. Birds of group B had higher relative weights of liver (P = 0.57) and gizzard (P = 0.69). The relative caecal weights of all MOS-supplemented groups were greater (P = 0.10) compared with the group A. Serum concentrations of calcium, magnesium, copper and iron were not influenced by the MOS supplementations. Serum phosphorus level was higher (P < 0.05) in the group B compared with the group A. The caecal Clostridium spp. counts were more (P = 0.001) in the group B compared with the other groups, while E. coli population remained unchanged in all groups. We found that supplementation of MOS partially influenced the growth performance of Japanese quails.