Coronary artery ectasia (CAE) in adults is often caused by atherosclerotic plaques. CAE can affect atherosclerotic plaques through hemodynamic changes. However, no study has evaluated the characteristics of CAE with atherosclerotic plaques. Therefore, we aimed to disclose the characteristics of atherosclerotic plaques in patients with CAE using optical coherence tomography (OCT). We evaluated patients with CAE, confirmed by coronary angiography, who underwent pre-intervention OCT between April 2015 and April 2021. Each millimeter of the OCT images was analyzed to assess the characteristics of CAEs, plaque phenotypes, and plaque vulnerability. A total of 286 patients (344 coronary vessels) met our criteria, 82.87% of whom were men. Right coronary artery lesions were the most common, comprising 44.48% (n = 153) of the total. We found 329 CAE vessels with plaques, accounting for 95.64% of the coronary vessels. After grouping CAEs and plaques by their relative positions, we found that the length of plaques within CAE lesions was longer than that of plaques in other sites (P < 0.001). Plaques within CAE lesions had greater maximum lipid angles and lipid indexes (P = 0.007, P = 0.004, respectively) than those on other sites. This study revealed the most common vascular and morphological characteristics of CAE. While the accompanying plaques were not affected by the location or morphology of the CAE vessels, they were affected by their position relative to the CAE lesion.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10554-023-02835-9.