Microvascular decompression (MVD) surgery has become the most definitive treatment for hemifacial spasm (HFS). Today, retromastoid craniectomy for MVD surgery is widely performed and standardized surgical technique has settled. Complications of this procedure are related to the injury of cranial nerves and vessels in the lower cerebellopontine angle. We experienced a rare case of abnormal running sigmoid sinus. Here, we must attend to the preoperative imaging studies such as MR images while surgical procedures are performed.