Formation of carotid artery pseudoaneurysm is uncommon after intensive treatment for head and neck cancer. We encountered a case of postoperative formation of pseudoaneurysm at the left carotid bifurcation in a diabetic man. The risk factors for carotid artery pseudoaneurysm in the patient included diabetes mellitus, previous radiotherapy to the neck, neck dissection, and postoperative Staphylococcus aureus infection secondary to accidental pharyngocutaneous fistula. We successfully obliterated this pseudoaneurysm using a Smart stent and detachable coils without neurological deficits. The possibility of vascular injury after treatment for head and neck cancer must be considered, especially in patients with risk factors for it due to previous treatment.