Diets supplemented with fish oil (FO), which is rich in n-3 PUFA, have been shown to modify several key risk factors for CVD. The purpose of the present study was to determine the effect of FO supplementation on mitochondrial dynamic protein expression in the endothelium and on endothelial cell function. Male apoE-deficient (apoE− / −) mice (8 weeks old, n 12 per group) were fed a high-fat diet containing 45 % fat (HFD group) or a HFD with partial replacement of lard with 10 % (w/w) FO (FO group) (total EPA and DHA content 64·1 g/kg) for 8 weeks. ApoE− / − mice in the FO group had a greater endothelium-dependent vasorelaxation response to acetylcholine (Ach) than those in the HFD group. The atherosclerotic lesion volume in the aortic sinus of mice in the FO group was 54 % lower than that in the HFD group (P< 0·01). In addition, the aortas isolated from mice in the FO group had higher expression levels of Mfn2 and Opa1 but lower expression levels of Fis1 than those from the HFD group. Compared with mice fed the HFD, those fed the FO diet showed significantly lower levels of mitochondrial oxidative stress, cytochrome c release and caspase-3 activity (each P< 0·05). Furthermore, FO-fed mice displayed increased NO release and availability and enhanced endothelial NO synthase activity compared with HFD-fed mice. Taken together, these results reveal a novel mechanism by which FO protects against endothelial cell dysfunction, which may result in improved mitochondrial dynamics.