Background: Endothelial dysfunction is associated with increased oxidative stress, an important promoter of inflammatory processes. The process of inflammation is regulated in part by nitric oxide. Thus, a dysfunctional endothelium may contribute to plaque destabilization and plaque erosion. The aim of study is to investigate the difference of endothelial dysfunction and arterial stiffness according to plaque morphology. Methods: Forty patients with unstable angina who underwent coronary angiography and intravascular ultrasound were divided into soft plaque group (n=18, 57.6짹8.9 years, 10 males) and nonsoft plaque group (n=22, 59.1짹8.1 years, 13 males). Flow mediated vasodilation (FMD) of brachial artery and pulse wave velocity (PWV) were measured before coronary angiography. Results: The baseline characteristics between the groups with soft plaque group and nonsoft plaque group are similar. FMD was significantly more decreased in soft plaque group compared with nonsoft plaque group (5.5짹3.8 vs. 9.1짹4.0 %, p=0.014). Nitrate mediated dilation was not different between the groups. Femoral-ankle PWV was significantly higher in the soft plaque group (1161.2짹111.9 vs. 1059.5짹88.6 cm/s, p=0.044) and positively correlated with NMD, lipoprotein (a), and monocyte count (r=0.562, p=0.029; r=664, p=0.026; r=0.628, p=0.012, respectively). Also, the level of high-sensitivity C-reactive protein (hsCRP), and fibrinogen were significantly higher in the soft plaque group (1.0짹0.7 vs. 0.3짹0.3 mg/dL, p=0.034; 302.3짹65.0 vs. 237.3짹91.7 mg/dL, p=0.041, respectively). Conclusion: Increased endothelial dysfunction and arterial stiffness may be associated with plaque destabilization.