Aim To investigate the vulnerable mechanism of mildly lesion by analyzing the structural characteristics of plaque. Methods In 62 patients with angiographic mildly coronary stenosis (diameter stenosis 20%～60%) and 26 patients with several stenosis, intravascular ultrasound (IVUS) imaging was performed. The characteristics of the atherosclerosis plaques were analyzed and stenosis rate was calculated. Results Compared with patients with angiographic several stenosis, patients with angiographic mildly lesion had more soft plaque (68.2% vs 15.4%, p<0.01), thin fibrosis cap (65.9% vs 7.7%, p< 0.01), plaque rupture ( 28.2% vs7.7%, p<0.01), and positive remodeling (51.8% vs 0%, p<0.01). Patients with mildly lesion had more eccentric plaques (84.7% vs 30.1%, p<0.01) and less calcification (16.5% vs 84.6%, p<0.01) than those with angiographic several stenosis. Conclusions In the majority of angiographic mildly coronary stenosis, plaques have several structural features of vulnerability. These features may be contributed to acute coronary syndrome.