The size of carotid atherosclerotic plaques is associated with ischemic cerebrovascular events. Both 3D MR vessel wall imaging and 2D ultrasound can measure carotid plaques. To improve the work flow of screening for subclinical carotid atherosclerosis, this study sought to compare the quantitative measurements of carotid plaque between 3D MR and 2D ultrasound imaging. Excellent agreement was found between MR and ultrasound imaging in measuring carotid artery maximum wall thickness. Although there was moderate to strong correlation between MR and ultrasound imaging, the plaque area measured by MR imaging was more than two folds than that measured by ultrasound imaging.