Low-grade myofibroblastic sarcoma is a rare disorder of malignant soft tissue tumor and is usually occurred various regions that often manifests in the head and neck region. The most common clinical presentation of this entity is non-tender cervical lymphadenopathy. This disease usually presents high possibility of local recurrence and low possibility of distant metastasis. We report a rare case of low-grade myofibroblastic sarcoma occurring in the sternocleidomastoid muscle and invading to the mastoid tip. A 56-year-old male visited the clinic with a complaint of slowly growing postauricular mass for 6 months. Mass originating from sternomastoid muscle and invasion to mastoid tip was observed by imaging studies. Surgical complete excision with simple mastoidectomy was performed via modified facelift approach. The histopathologic findings revealed malignant spindle cell tumor with positive staining with smooth muscle actin. The patient has no recurrence for 2years without any complication.