Orbital mass is about 17% of orbital disease and only 7-8% of mass is lymphoma. Lymphoma involved extraocular muscles are unusual. Lymphoma occurred in lateral rectus muscle, rarely. The authors experienced a case of malignant lymphoma occurred in the lateral rectus muscle in 17-year-old male. His main symptom was exopthalmos and diplopia. The patient was treated with excision via bicoronal approach and radiation. We report the unusual case of malignant lymphoma occurred in lateral rectus muscle.