Mannan oligosaccharide is a new kind of feed additive whose effects on fish flesh quality has not been well studied. In this study, 315 healthy juvenile Oreochromis niloticus with an average body weight of (2.2±0.2) g were randomly divided into three treatment groups: control group (C, 35% carbohydrate), high carbohydrate diet group (HC, 45% carbohydrate) and high carbohydrate diet supplemented with mannan-oligosaccharide (HM, 45% carbohydrate, 0.5% mannan oligosaccharide). Three tanks for each group, and 35 fish reared in each tank. After feeding trial for 10 weeks, the growth indicator, muscle nutritional composition and flesh texture of all groups were determined. Compared with the control group, the hepatosomatic index, muscle cohesiveness, myofiber number, content of inosinic acid and triglyceride, proportion of delicious amino acids, and elative expression of CAST gene, which could improve muscle growth, were significantly increased in the fish of HC group; the muscle hardness, gumminess and chewiness, myofiber diameter, cooking loss, proportion of essential amino acids, contents of fatty acids, diglyceride and phosphatidylinositol, and relative expression of MyoG gene, which functions in the differentiation of muscle fibers, were significantly decreased in the fish of HC group. Compared with the HC group, the muscle springiness, proportion of essential amino acids, contents of fatty acids and diglyceride, and CAST gene relative expression were significantly increased in O. niloticus of HM group; the muscle adhesion, number of myofiber, contents of inosinic acid, phosphatidylethanolamine and phosphatidyl choline, and MyoG gene relative expression in O. niloticus of HM group were significantly decreased. Compared with the control group, the weight gain rate, muscle cohesion and contents of triglyceride and fatty acids were significantly increased in fish of the HM group; the hepatosomatic index, muscle hardness and chewiness, content of phospholipid in the fish of HM group were significantly decreased. The above results indicated that with the addition of manna oligosaccharides in high carbohydrate diet, the meat yield of O. niloticus was greatly improved, the flesh quality was improved due to elevated water holding capacity, texture and nutritional value of amino acids, but the decreased phospholipid content should not be neglected. In this study, the effects of mannan oligosaccharides on the growth and muscle quality of O. niloticus were comprehensively evaluated, with a view to broadening its application as a feed additive.