Aim To investigate the endothelial function in patients with acute coronary syndrome (ACS) and the effect of percutaneous coronary intervention (PCI) therapy on endothelial function. Methods Endothelium- dependent flow-mediated dilation (FMD)and endothelium-independent nitroglycein-mediated dilation (NMD)of brachial artery were assessed by high-resolution color Doppler ultrosound. One hundred and twenty patients with ACS and thirty control subjects were enrolled in the study. First, the groups were matched for age, sex and other risk factors, people with hypertension and diabetes were excluded. FMD and NMD of two groups were compared. Second, basline FMD and NMD of all patients were measured. Then, patients were divided into PCI and medical groups. FMD and NMD for basline and three months later of two groups were compared. Results Compared with healthy controls, ACS patients had lower FMD (8.29±5.11 vs 10.64±3.82, P=0.029), but equal NMD (20.37±9.29 vs 18.41±5.83, P=0.226); For PCI group, FMD of basline and three months later was (7.86±5.51, 5.26±7.20), respectively (P=0.037); while of medical group, FMD of basline and three months later was (7.91± 4.52 , 7.14±6.99), respectively (P=0.401), No significant difference was found in NMD between two groups, before and after therapy (p>0.05). Conclusions The endothelial function was impaired in ACS patients; The treatment of PCI augment endothelial dysfunction.