A case of desmoplastic fibroma of mandible in a 18 years old woman is presented. She had complained progressive swelling of right mandible for 4 years. Radiographically, a multilocular radiolucent of right hemimandibulectomy showed multinodular external surface without cortical destruction. Cut surfaces revealed grayish white, fibrous homogeneous appearance with firm consistency, sparing head portion. The maximum diameter of the tumor was 13 cm. Microscopically, the tumor was composed of interlacing bundles of monomorphic spindle-shaped cells with abundant intercellular collagen. Ultrastructurally, most tumor cells were fibroblastic-like cells with abundant RER and cytoplasmic fibrils, but a few disclosed transition to myofibroblasts. However, no fully developed myofibroblasts were seen.