Aim To identify the characteristic of unstable atherosclerotic plaques in acute coronary syndrome with intravascular ultrasound (IVUS). Methods Thirty-six patients with acute coronary syndrome and twenty patients with stable angina pectoris underwent coronary angiography (CAG) and IVUS examinations respectively. IVUS was used to analyze the qualities of the plaques and then measure the external elastic membrane (EEM) area, lumen area (LA), plaque area (PA) and lumen area narrowing at both the lesion and the reference segments in the same vessel respectively, and lumen eccentricity index and remodeling index were calculated. The characteristic of unstable atherosclerotic plaques were derived by comparing the results of IVUS in the stable and unstable plaques. Results 77.8%(28/36)patients with acute coronary syndrome had soft lipid plaques, ten lesions in ACS patients had plaque rupture and thrombosis; while patients with stable angina pectoris mainly had fibrous and mixed plaques, only 10%(2/20)had soft plaques. There were distinctive difference in eccentric plaques, EEM area, plaque area and lumen area narrowing between the two groups. Positive remodeling pattern was observed in 72.2%(26/36) unstable plaques while 75%(15/20) stable plaques showed negative remodeling. Conclusions IVUS can detect unstable atherosclerotic plaques and predict plaques disruption in early stage of atherosclerosis.