It is essential to define the accurate location and extent of tumors for the treatment planning and evaluation of the prognosis of the musculoskeletal tumors. Since Magnetic Resonance Imaging(MRI) provides excellent tissue contrast and multiplsnar image. MRI could be applied to the diagnosis and the treatment planning of musculo-skeletal tumors. Thirty-one patients with bone and soft tissue tumors underwent MRI at Department of Orthopedic Surgery, Seoul National University Hospital, since Jun. 1986, and the applicability of MRI in treatment planning of bone and soft tissue tumors was evaluated. Multiplanar image and excellent tissue contrast between soft tissue made it possible to define the location and the extent of tumor accurately. Tl weighted image was advantageous in intramedullary tumor extent, while T2 weighted image was helpful in soft tissue invasion. It was possible to guess histologic diagnosis of tumor in such cases as hemangioma, aneurysmal bone cyst, lipoma, and desmoid tumor. Above advantages guided the treatment planning and determination of biopsy site. Especially in two cases of osteosarcoma, which were highly malignant, limb salvage operation was performed instead of amputation.