Background: Early spontaneous reperfusion (ESR) is not an uncommon phenomenon in clinical settings. Aims: The aim of this study was to detect potential mechanisms of ESR in patients with STEMI. Methods: This prospective study enrolled a total of 241 consecutive patients with STEMI undergoing opti-cal coherence tomography (OCT) from July 2016 to August 2019. Forty-five patients (18.7%) met angio-graphic ESR criteria (TIMI 3 flow on the initial angiogram). Among those without ESR (TIMI 0 flow on initial angiogram), 45 patients were assigned to the control group according to propensity score matching with the ESR group. Results: Although the baseline characteristics of the groups were comparable, non-ruptured plaque (62.2% vs 35.6%) predominated and plaque rupture (37.8% vs 64.4%) was less common in the ESR group (p=0.011). Red thrombus (44.4% vs 77.8%) was also less common in the ESR group (p=0.001). Lastly, compared to the control group, the ESR group underwent fewer emergent stent placements (68.9% vs 91.1%, p=0.008). Conclusions: Relief of coronary occlusion induced by a non-ruptured plaque may contribute to ESR in patients with STEMI.