A 68-year-old male patient presented with a week of sudden diplopia. He had been diagnosed with nasopharyngeal cancer 8 months prior and had undergone chemotherapy with radiotherapy. Eight-prism diopter right esotropia in the primary position and a remarkable limitation in abduction in his right eye were observed. Other pupillary disorders and lid drooping were not found. After three weeks, the marginal reflex distance 1 was 3 mm in the right eye and 5 mm in the left eye. The pupil diameter was 2.5 mm in the right eye, and 3 mm in the left eye under room illumination. Under darkened conditions, the pupil diameter was 3.5 mm in the right eye, and 5 mm in the left eye. After topical application of 0.5% apraclonidine, improvement in the right ptosis and reversal pupillary dilatation were observed. On brain magnetic resonance imaging, enhanced lesions on the right cavernous sinus, both sphenoidal sinuses, and skull base suggested the invasion of nasopharyngeal cancer. Lesions on the cavernous sinus need to be considered in cases of abducens nerve palsy and ipsilateral Horner's syndrome.