Abstract: AIM:To evaluate the characteristics of the culprit lesion in acute myocardial infarction and stable angina pectoris by optical coherence tomography (OCT). METHODS: A total of 140 patients undergoing coronary artery angiography (CAG) and OCT were enrolled, including 61 cases (55.3±11.8) of acute ST segment elevation myocardial infarction (AMI) and 79 cases (59.0±9.6) of stable angina pectoris (SAP). Lesion characteristics were assessed by OCT and related clinical data were analyzed. RESULTS: The minimum thickness of the fibrous cap in AMI group was significantly thinner than in SAP group ［(45±13） μm vs. （80±39） μm, P<0.01］ and the thin cap fibroatheroma (TCFA) in AMI group was higher than in SAP group ［33 (56%) vs. 17 (25%), P<0.01］. Thrombi were more frequently observed in AMI group ［32 (52%) vs. 4 (5%), P<0.01］ and plaque rupture was 41 (67%) in AMI group and 20 (25%) in SAP group (P<0.01). AMI patients had a higher incidence of moderate and severe plaque disruption (ulcer+dissection) than SAP patients and lipidrich plaques (=4 quadrants) were more frequently observed in AMI group. No significant difference was found in the depth and extent of calcification between groups, but calcified plaques (≥2 quadrants) were found more in SAP group. CONCLUSION: OCT can accurately evaluate the characteristics and structure of the culprit lesion in acute myocardial infarction. Thin cap fibroatheromas, more severe plaque disruptions, thrombi and vulnerable plaques are more frequently observed in AMI patients than in SAP patients.