To investigate the effects of high niacin levels on lipid metabolism and body fat deposition in fish, two experiments on genetically improved farmed tilapia (GIFT, ) were designed. In experiment I, to determine whether high niacin can decrease serum lipid content in fish and its appropriate level, a model of hyperlipidemia was established by first feeding GIFT (initial body weight:15.28±0.23 g) a high-carbohydrate and high-fat diet for 40 d; subsequently, these fish were fed diets containing 500 mg/kg, 1000 mg/kg, and 2000 mg/kg niacin. The results showed that the addition of 500-2000 mg/kg niacin in the diet could reduce fish serum cholesterol and triglyceride levels, and that 1000-2000 mg/kg niacin had more highly significant effects. In experimental Ⅱ, a 2×2 factorial study was conducted to investigate the effects of two diet types (the control diet and high-carbohydrate and high-fat diet) and two niacin levels (100 mg/kg and 1000 mg/kg) on the growth, feed utilization, serum lipid level, and body composition of GIFT (initial body weight:24.45±0.07 g). The experiment lasted for 8 weeks. The results showed that neither of the diet types nor niacin levels had significant effects ( > 0.05) on the final mean weight, weight gain, specific growth rate, feed conversion ratio, feeding intake, condition factor, viscerosomatic index, mesenteric fat index, or the apparent digestibility of crude protein and crude fat, or crude fat, crude protein, moisture, and ash content of the whole fish. Apparent digestibility of dry matter and energy, and the levels of serum cholesterol, triglycerides and free fatty acid (FFA), and the crude fat contents of liver, viscera, and abdominal muscle of fish fed the high-carbohydrate and high-fat diet were significantly higher ( < 0.05) than those of the control group. The serum low density lipoprotein, cholesterol and FFA, and liver crude fat contents of fish fed the diets contain 1000 mg/kg niacin were significantly lower ( < 0.05) than those fed the diets containing 100 mg/kg niacin. There were significant interactions ( < 0.05) between dietary types and niacin levels on serum high density lipoprotein and cholesterol contents. These results indicated that the supplementation of 1000 mg/kg niacin in the diet would not affect the growth performance and feed utilization of GIFT. However, it could reduce the levels of serum lipids and liver fat content.