A periosteal reaction is a subperiosteal new bone that is formed when the periosteum is stimulated and causes periosteal hyperplasia. The presence of periosteal reaction indicates the presence of disease in the bone or body and requires prompt medical examination to find the primary disease causing the periosteal reaction and to provide aggressive treatment. The most common diseases that cause periosteal reactions are acute and chronic osteomyelitis, bone tuberculosis, and benign and malignant bone tumors such as osteolytic osteosarcoma and Ewing’s sarcoma. All of the above diseases can cause periosteal reaction, and the general disease can be determined by the general pattern of periosteal reaction through X-ray examination. For example, the osteochondral reaction on X-ray is a triangular high-density shadow, a manifestation known as Codman’s triangle, which is an imaging manifestation of osteosarcoma. If this occurs, the possibility of osteosarcoma is highly suspected and requires aggressive treatment.