Culprit lesions of acute coronary syndrome (ACS) were observed by intravascular optical coherence tomography (OCT). OCT images revealed diffuse intimal thickening, reduced lumen area with vascular contraction, and thrombus formation. No OCT images of atherosclerotic plaque disruption were found. Vascular contraction disappeared and the lumen was dilated after intracoronary injection of nitroglycerin. The main mechanism of ACS in this case was therefore considered to be coronary vasospasm. OCT may be useful for evaluating the mechanism of ACS.