The purpose of this study is to determine the effects of Cyclophosphamide on the healing process of fractures of the mice. Tweny-one young adult male mice were used. All of them were experimentally fractured in the left mandibular body areas, and Cyclophosphamide 0.1mg. per 30gr. body wt. was administrated intramuscularly to the experimental group. the mice of both control and experimental groups were sacrificed 1,3,5,7,10,14, and 21st day after operations and microscopic slides were made. The author has observed the histopathological findings. The results were as follows; 1. There were no specific difference between the experimental and control group in the early stage (1~5th day after operation) of the healing process of the mandibular fracture. 2. In the healing process of fractures 7-10th day after operation, the fibrous tissure formation and osteoblastic activity were poor in the experimental groop compared with the control group. 3. In the healing process of fractures 14-21th day after operation, the connective tissue and new bone formation were very poor in the experimental group compared with the control group. 4. On the whole, cyclophosphamide affected the experimentally fractured wound to delay healing in the jaw bone of the mice.