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 an...