Synovial sarcoma is the fourth most common type of soft-tissue sarcoma, accounting for 2.5%-10.5% of all primary soft-tissue neoplasms worldwide. This disease most often affects the extremities, 80%-95% of cases, of adolescents and young adults (15-40 years of age). Despite its name, the lesion does not commonly arise in an intraarticular location but usually occurs near joints. Etiology is still unclear. Synovial sarcoma is an intermediate-to high-grade neoplasm with poor prognosis, and, despite initial aggressive wide surgical resection, local recurrence and metastasis are common. Understanding and recognizing the spectrum of appearances of this malignancy, which reflect the underlying pathologic characteristics, improve radiologic evaluation are essential for optimal patient management. A twenty year-old girl admitted to our institution with an rapid increasing mass on her left thigh recently. The mass was first noted 5 years ago. Imaging studies including X ray, ultrasound and MRI suggested a soft tissue sarcoma. Definite diagnosis of monophasic synovial sarcoma, fibrous type was made based on histopathological findings.