Thymomas are rare tumors that commonly lie in the anterior mediastinum. The arteries supplying thymomas branch from the internal thoracic arteries and their collaterals. This report presents a patient with huge mediastinal thymoma, whose sensational vascularization is ensured directly by a single dominant tributary branch of the left internal thoracic artery. After initially performed computed tomography angiography, the tumor was suspect to be vascularized from the right coronary artery. Triple-rule-out computed tomography scan was a key step for accurate radiologic diagnosis.