Abstract: To investigate effects of oxidized fish oil on the structure and permeability of intestine 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, respectively. Compared with the 6% soybean oil, the oxidative product of fish oil at low level increased serum and intestine MDA and GSH content but diminished the GSH content at high level. The oxidized fish oil significantly reduced intestine bile acid content (P0.05). The oxidized fish oil enhanced the number of goblet cell (P0.05), The microvilli of intestine was enlarged by low oxidized fish oil level and atrophied by high level. Serum D-lactic acid and endotoxin content increased significantly by oxidized fish oil because of the enlarged gap between tight junction, the hyperplasia and edema of villi. Thus, the oxidized fish oil might damage the intestinal epithelial tight junction and destroy the intestinal mucosal epithelium, which would increase intestinal oxidative stress and interfere the bile acids-cholesterol circulation of grass carp.