Masseter muscle hypertrophy is characterized by unilateral or bilateral enlargement of the masseter muscles affecting both females and males after puberty. Limitations on mouth opening, swollen cheek, and also tension in the region of the hypertrophied muscle are symptoms reported. Also, masseter hypertrophy can cause aesthetic and functional problems. A 40-year old woman was referred to our clinic with the chief complaint of facial appearance with square-face type. To eliminate undesirable facial appearance, surgical intraoral approach compromising reduction of deep masseter muscle with monocortical and bicortical ostectomy of the angle of the mandible was performed. The patient was satisfied with both functional outcomes and aesthetic outcomes on both facial profile and frontal view. No complication was seen intraoperatively and postoperatively after a 12-month follow-up period. This treatment modality would be suggested to gain optimal aesthetic results especially in a square face from the lateral profile.