We investigated the effects of dietary β-1,4-mannobiose (MNB) on the weights of body, breast muscles, thighs, liver, and abdominal adipose tissue in growing broiler chicks to evaluate whether MNB can be used as a feed additive for broiler chicks. A total of 24 eight day-old male broiler chicks were allocated to two cages and fed a control diet or a 0.01% MNB-supplemented diet for 14 days. Dietary MNB significantly increased the relative weight of breast muscles, whereas the weights of the body, thighs, liver and abdominal adipose tissue were not affected. The myostatin mRNA level in the breast muscle was significantly reduced by MNB. Since myostatin is a negative regulator of myoblast proliferation and differentiation, it is possible that the downregulation of myostatin gene expression is involved in the increased breast muscle growth with MNB. The plasma 3-methyl histidine level, which is known to be a nonmetabolizable amino acid marker of myofibrillar protein catabolism, and the breast muscle atrogin-1 mRNA level, which is involved in protein catabolism, were not affected by dietary MNB. In addition, MNB did not affect protease mRNA levels in the breast muscles. These results suggest that MNB does not affect proteolysis in the breast muscles. All our findings suggest that MNB is a promising candidate feed additive to improve the meat yield of broiler chickens.