Prevention and management of coronary artery disease (CAD) is of great concern in patients with diabetes mellitus. Although the impact of coronary atherosclerosis is described well for subjects older than 40 years, the prevalence and types of coronary atherosclerosis in young patients are not well known. The aim of this study was to evaluate the prevalence, extent, severity, and volumes of coronary plaque in type 2 diabetes mellitus (T2DM) population younger than of 40 years. This prospective study enrolled 181 subjects (25-40 year old) undergoing coronary computed tomography angiography, with 86 T2DM and 95 nondiabetic age/gender-matched subjects. Coronary artery calcium (CAC), plaque assessment including total segment stenosis (sum of individual segmental stenosis), total plaque scores (sum of semiquantitative segmental plaque burden), segment involvement scores (number of segments with plaque) were evaluated. In addition, we quantitatively measured plaque volumes in total, fibrous, fibrous fatty, dense calcified, and low-attenuation plaque using novel plaque software. Compared with nondiabetic patients, the prevalence of CAD, calcified, and noncalcified plaques was higher in patients with T2DM (19% vs 58%; p <0.001). In patients with a zero CAC, T2DM had a higher prevalence (46%) of noncalcified plaque (p <0.0001). In multivariate linear regression models after adjusting for traditional cardiovascular risk factors, increased total segmental stenosis, total plaque scores, and segment involvement scores were associated with T2DM. Regarding quantitative plaque assessment, all volumes in noncalcified plaque type were approximately threefold higher in patients with T2DM. In conclusion, young patients with T2DM are susceptible to premature CAD with more calcified and noncalcified plaques. Early prevention program using computed tomography angiography might be helpful in identifying young diabetic patients with subclinical atherosclerosis.