We describe a case of bone tumor in the left tenth rib that was diagnosed as a low‐grade osteosarcoma resembling osteoblastoma. This diagnosis was supported by clinical, radiologic, and histologic findings. Specimens of this tumor were transplanted into nude mice, and the morphology of the transplanted tumors was examined. The transplanted tumors were similar histologically to the parent tumor. Ultrastructurally, the transplanted osteoblasts showed irregular, indented nuclei, dilated endoplasmic reticulum, and varying amounts of intercellular junctional complexes. Our transplantable tumor could be valuable as an experimental model for studies on this tumor typ