A 72-day experiment was carried out to investigate the effect of oxidized fish oil on intestinal mucosa barrier structure and intestinal permeability of grass carp (Ctenopharyngodon idella)[average weight (74.8±1.2) g].Five diets containing different levels of soybean oil,fish oil and oxidized fish oil (6% fish oil,6% soybean oil,2% oxidized fish oil; 4% oxidized fish oil,6% oxidized fish oil) were evaluated according to the equal standards of nitrogen and energy.Using qRT-PCR method,the gene expression levels about atresia protein Occludin,closed protein Claudin3,Claudin15a and cytoplasmic protein ZO-1,ZO-2,ZO-3 in the intestine were measured,and TEM observations of intestinal mucosal cells tight junction and intestinal permeability index were comprehensively analyzed.The results showed that,(1) Serum diamine oxidase activity,endotoxin and D-lactic acid content significantly increased under oxidized fish oil (P<0.05).(2) The gene expression of closed protein Claudin-3,Claudin-15a and cytoplasmic protein ZO-1,ZO-2,ZO-3 were significantly decreased under oxidized fish oil (P<0.05).(3) The tight junction structure of intestinal mucosal cells displayed gap under oxidized fish oil.(4) The values of AV,POV and the content of MDA in diet downregulated with closed protein Claudin3,Claudin15a and cytoplasmic protein ZO-1,ZO-2,ZO-3 gene expression,besides, the value of POV and the content of MDA in diet significantly downregulated with cytoplasmic protein ZO-1 gene expression under oxidized fish oil (P<0.05).Collectively,the present study demonstrated that after adding oxidized fish oil in feed,peroxide oxidation and other oil oxidation product like malondialdehyde of fish oil led to the fact that tight junction protein gene expression activity significantly decreased and tight junction physical structure significant damaged, resulting in the fact that intestinal barrier structure was destroyed and intestinal permeability significantly increased.