Abstract: A 65-day trial was conducted to investigate effects of CLA on cytological morphology, activity of antioxidant enzymes, and gene expressions of lipid metabolism in the liver and muscle of grass carp. Seven isonitrogenous (crude protein: 36 g/100 g) and isolipidic (crude lipid: 4.5 g/100 g) diets were formulated: 0 CLA (control), 0.5% (CLA0.5), 1% (CLA1), 1.5 (CLA1.5), 2 (CLA2), 2.5 (CLA2.5), and 3% CLA (CLA3). Three biological replicates of fish were assigned to each diet with the initial body weight at (5.08±0.08) g. Endoplasmic reticulum in the liver, and loose sarcolemma of myocyte and degradation of myofibril in the muscle when compared with the control fish. The activities of superoxide dismutase (SOD) and glutathione peroxidase (GPx), and the total antioxidant capacity (T-AOC) both in the liver and muscle of CLA1.5-CLA2.5 groups significantly (P<0.05) increased compared with control group. The activities of catalase (CAT) in the liver of CLA1.5-CLA3 groups were significantly (P<0.05) higher than that of control group, however, there was no significant (P>0.05) change in the glutathione reductase (GR) activity in the liver. The MDA contents in the liver of fish fed with CLA1.5-CLA2 diets were significantly (P<0.05) lower than that of control group, however, the MDA contents in the liver of fish fed with CLA2.5-CLA3 diets significantly (P<0.05) increased compared with control group. In contrast, fish fed with CLA3 had significantly higher MDA contents in the muscle (P<0.05) compared with control group. Compared with the control fish, the mRNA level of acetyl-CoA carboxylase (ACC) both in the liver and muscle of fish fed with CLA1.5-CLA2.5 diets were significantly (P<0.05) down-regulated; the mRNA levels of lipoprotein lipase (LPL), hormone-sensitive lipase (HSL), and peroxisome proliferator-activated receptor alpha (PPARα) of fish fed with CLA1.5-CLA2.5 diets were significantly (P<0.05) up-regulated; the mRNA levels of fatty acid desaturase 2 (FAD2) in the liver of fish fed with CLA2-CLA3 diets and in the muscle of fish fed with CLA1.5-CLA2 and CLA3 diets were significantly (P<0.05) up-regulated; the mRNA level of peroxisome proliferator-activated receptor gamma (PPARγ) in the liver of fish fed with CLA1-CLA3 diets was significantly (P<0.05) down-regulated. Conversely, there was no significant (P>0.05) difference for the PPARγ mRNA level in the muscle of fish fed with CLA0.5-CLA3 diets. In conclusion, supplementation of 2% CLA could significantly decrease the lipid contents in the liver and muscle without affecting cytological morphology of liver and muscle, the growth and feed utilization of grass carp by improving the antioxidant capacities in the liver and muscle.