Objective Epicardial adipose tissue (EAT) is suggested to play an important role in progression of metabolic syndrome (MS). We aimed to examine relations between EAT thickness and a single-slice epicardial fat area (EFA) measured by multidetector computed tomography (MDCT), and changes of EAT thickness according to the presence of type 2 diabetes mellitus (T2DM), and obesity. Methods A total of 94 patients (42.6% DM, 53.2% obese, mean age 60.7 ± 13.2) who underwent MDCT were enrolled. Thickness of EAT was measured on the parasternal short and horizontal long axis view. EFA was measured at the level of left main coronary artery (LMCA). Results All EAT thicknesses were correlated with EFA at the LMCA level (r = 0.235~0.613, all Ps < 0.05), and EAT thickness in the left atrioventricular groove (LAVG) had the highest correlation coefficient (r = 0.613). Subjects without T2DM and obesity had lower values of EAT thickness in LAVG than subjects with T2DM or obesity. When adjusted for body mass index (BMI), values of EAT thickness in LAVG were higher in subjects with T2DM than subjects without T2DM (P < 0.05). Multiple linear regression analysis revealed that age, T2DM, and BMI were independent variables related to EAT thickness in LAVG (all Ps < 0.05). Conclusion We demonstrated that EAT thickness measured by MDCT (especially LAVG) is well correlated with EFA. The EAT thickness in LAVG was associated with BMI. Also, increased EAT thickness was associated with T2DM even when they are not obese. These findings implicate that EAT thickness in LAVG may be a useful indicator for T2DM and obesity independently.