INTRODUCTION: Diabetes mellitus (DM) negatively affects fracture repair by inhibiting endochondral ossification, chondrogenesis, callus formation, and angiogenesis. We previously reported that transcutaneous CO2 application accelerates fracture repair by promoting endochondral ossification and angiogenesis. The present study aimed to determine whether CO2 treatment would promote fracture repair in cases with type I DM. RESEARCH DESIGN AND METHODS: A closed femoral shaft fracture was induced in female rats with streptozotocin-induced type I DM. CO2 treatment was performed five times a week for the CO2 group. Sham treatment, where CO2 was replaced with air, was performed for the control group. Radiographic, histologic, genetic, and biomechanical measurements were taken at several time points. RESULTS: Radiographic assessment demonstrated that fracture repair was induced in the CO2 group. Histologically, accelerated endochondral ossification and capillary formation were observed in the CO2 group. Immunohistochemical assessment indicated that early postfracture proliferation of chondrocytes in callus was enhanced in the CO2 group. Genetic assessment results suggested that cartilage and bone formation, angiogenesis, and vasodilation were upregulated in the CO2 group. Biomechanical assessment revealed enhanced mechanical strength in the CO2 group. CONCLUSIONS: Our findings suggest that CO2 treatment accelerates fracture repair in type I DM rats. CO2 treatment could be an effective strategy for delayed fracture repair due to DM.