Abstract: To investigate effects of oxidized fish oil on the structure and permeability of hepatopancreas of grass carp (Ctenopharyngodon idellus), five iso-nitrogen and iso-energy diets were formulated with soybean oil, fish oil and oxidized fish oil at different concentrations, 6% fish oil, 6% soybean oil, 2% oxidized fish oil and 4% soybean oil, 4% oxidized fish oil and 2% soybean oil, 6% oxidized fish oil for a 72d experiment. Result showed that oxidized fish oil significantly increased (P0.05) the content of ALB, GLB, MDA, GSH of grass carp serum and significantly reduced (P0.05) the content of GSH, SOD of grass carp hepatopancreas. Oxidized fish oil significantly increased (P0.05) the HSI, content of lipid of hepatopancreas and TG content of serum, and significantly reduced (P0.05) the ratio of HDL and LDL at the same time. Oxidized fish oil significantly enhanced the content of TC on serum and hepatopancreas and the content of TBA on hepatopancreas (P0.05) but significantly diminished the content of serum TBA (P0.05). Oxidized fish oil caused fatty liver by disrupting the fat metabolism, mitochondrial damage of hepatopancreas by impacting bile acid accumulation, hepatopancreas cell fibrosis, and tissue atrophyin grass carp. In conclusion, products of oxidized fish oil may cause grass carp oxidative stress and decrease antioxidant capacity of grass carp hepatopancreas.