Aim To study the changes in endothelium dependent vasodilation function,serum nitric oxide (NO) and superoxide dismutase (SOD) and their inter relationships in patients with essential hypertension (EH). Methods High resolution B mode ultrasonography was used to measure the endothelial dependent vasodilation in 28 patients with EH and 28 matched healthy subjects. Serum levels of NO and SOD were also measured. Results The increased percentage of the brachial artery diameter after reactive hyperemia was lower in patients with EH than that of normal controls (6.33%±2.59% vs 13.14%±4.24% p<0.001), whereas nitroglycerin induced changes were similar (p>0.05). Serum levels of NO,SOD in patients with EH was significantly lower compared with those in controls (53.55±23.5 μmol/L vs 83.26±23.25 μmol/L,86.57±26.66 kNU/L vs 117.1±33.64 kNU/L,p<0.001). Correlation analysis shows that endothelium dependent vasodilation function was negatively correlated with ages,systolic blood pressure,dilated blood pressure,the coefficients were -0.366 (p<0.01),-0.65 (p<0.001),-0.672 (p<0.001),and was positively correlated with serum NO and SOD,the coefficients were 0.838 (p<0.001),0.683 (p<0.001). The analysis of multiple stepwise regression shows that the endothelium dependent vasodilation function was negatively correlated with dilated blood pressure and positively correlated with serum NO and SOD. Conclusion Our findings support that endothelium vasodilation is impaired and was correlated with reduction of NO and increase of active oxygen species.