The main objectives of this study was to observe the effects of hyperbaric oxygen therapy on the healing processes of mandibular fracture of streptozotocin-induced diabetic rats. Author used 60 rats (Sprague-Dawley Strain) deviding into control(30) and experimental groups(30). Complete fracture was produced on the left mandibular body of 60rats, rendered hyperbaric oxygen therapy (2 hrs. daily at 2.5 atm.) on experimental group and observed effects of hyperbaric oxygen therapy by microscopically. The obtained results were as follows; 1. Infiltration of inflammatory cells was no significant differences between the control and experimental group until 3rd week, but experimental group showed decreasing tendency after 4th week. 2. Severe proliferation of fibroblasts showed rather rapider in experimental group, at 2nd week, while at 3rd week in control group. 3. Osteoclasts appeared at 1st week in experimental group while at 3rd week in control group, and experimental group showed early bone resorption pattern. 4. Osteoblasts appeared at 1st week in experimental group while at 3rd week in control group, and experimental group showed prominent osteoblastic activity. 5. Moderate proliferation of capillary blood vessels showed in initial stage of experimental group while mild proliferation at 1-2nd week in control group. 6. Formation of cartilaginous callus showed at 4th week in experimental group, while at 6th week in control group. 7. Formation of bony callus showed mildly at 5th week, and moderately at 6th week in experimental group, while no appearance in control group, but complete bony union was not observed even in experimental group throughout this experiment.