We evaluated the efficacy and safety of the fixed dose of 5000 IU unfractionated heparin (UFH) represented as peak activated clotting time (ACT) according to the body mass index (BMI) tertiles in patients undergoing diagnostic transradial coronary angiography (TRCA). A total of 422 patients were included in the present study, 84 in the normal weight group, 218 in the overweight group, and the 120 in the grades 1 and 2 obesity groups. Radial artery occlusion (RAO) was observed in 29 (6.8%) patients and the hematoma was observed in 43 (10.1%) patients. The rate of RAO and hematoma did not differ across the BMI tertiles (P = .749 and P = .066). Also, peak ACT and procedure duration did not differ between the study groups (P = .703 and P = .999). The only independent predictor of hematoma was sheath/radial artery diameter (P = .011) and the independent predictors for RAO were peak ACT, sheath/radial artery diameter, and procedure duration (P = .001, P = .028, and P < .001, respectively). In conclusion, a fixed dose of 5000 IU UFH is safe and effective regardless of the BMI in diagnostic TRCA procedure.