Background: Small vessel disease (SVD) widely exists in patients with acute coronary syndrome. However, the plaque characteristic of SVD has not been investigated. Methods: Optical coherence tomography (OCT) of culprit lesion was examined in 576 patients with ST-segment elevation myocardial infarction (STEMI) and finally 404 patients with qualified images were analysed of plaque phenotypes and microstructure. The cohort was divided into three groups according to vessel diameters of culprit lesion which were measured by OCT. Major adverse cardiac events (MACEs) were recorded of each patient and compared among patients with different vessel diameters and plaque phenotypes. Results: Gender, age and body mass index (BMI) were significantly different among patients with different diameters of culprit vessels (98.4% vs. 85.7% vs.71.4%, p 0.001; 40.0 \pm 7.0 vs. 54.9 \pm 6.6 vs. 68.9 \pm 5.8, p 0.001; 28.4 \pm 4.0 vs. 25.8 \pm 2.9 vs. 25.2 \pm 3.0, p 0.001, respectively). Moreover, patients with diameters of culprit lesion >3 mm presented with more incidence of plaque rupture and macrophage (57.7% vs. 42.1% vs. 46.2%, p = 0.015, 55.1% vs. 41.0% vs. 36.9%, p = 0.010). Total MACE did not differ among groups of different vessel diameters and plaque phenotypes. Conclusions: Vessel size of culprit lesion is significantly associated with plaque phenotype in patients with STEMI. However, patients with different diameters and plaque phenotypes showed no significant difference of clinical outcomes. Clinical Trial Registration: NCT03593928.