Aim To investigate the protective role of n-3 polyunsaturated fatty acids on endothelial function.Methods The endothelium-dependent-relaxation (EDR) experiment is investigated in thoracic aorta ring of rat's hypercholesterolemia. Functional tests of nitric oxide synthase (NOS) and smooth muscle cell were done to illustrate the mechanism.Results Results indicate that, the impaired EDR under hypercholesterolemia is due to the incapable of NOS, n-3 polyunsaturated fatty acid can greatly improves the relaxation through recover the function of NOS.Conclusions One of the mechanism of beneficial role of n-3 polyunsaturated fatty acids on atherosclerosis is its protective function on endothelium.