A 62-year-old man with untreated diabetes complained of diplopia and headache. Neurological examination demonstrated left abducens nerve palsy. MRI showed a mass lesion in the left orbital apex. Total left ophthalmoplegia and visual loss rapidly developed in the next two weeks. A craniotomy was performed to decompress the orbital apex and remove the mass. The optic nerve was tightly encased by fibrous tissue. The pathological diagnosis was mucormycosis. Systemic administration of amphotericin B and fluconazole was started immediately. But the lesion rapidly invaded the cavernous sinus and occluded the left internal carotid artery. Finally, the patient died with intracranial extension of mucormycosis four months after the operation. Rhinocerebral mucormycosis is a rapidly progressive fatal disease. Successful treatment seems to be based on early diagnosis, control of the underlying disease, radical surgical resection, and systemic administration of amphotericin B. Mucormycosis should be considered as a differential diagnosis of orbital apex syndrome.