OBJECTIVE To study the changes of biomechanical properties and microarchitecture in callus during the healing process of osteoporotic fracture. METHODS Ovariectomy (OVX) was performed on SD rats to establish osteoporotic rat model. Osteotomy was performed on both groups of rats with OVX and sham operations to establish bone fracture. Bone mineral density (BMD), area of type I collagen, biomechanical properties, and microarchitecture of the fractured bones during healing process were measured. RESULTS The BMD of both groups of rats were significantly lower at the 12th week than at the 6th week after fracture (P < 0.05). Both groups of rats had significantly higher biomechanical strength and area of type I collagen at the 12th week than at the 6th week after fracture (P < 0.05). The OVX group of rats had significantly lower structure model index (SMI) values than the sham group at both 6th and 12th week after fracture (P < 0.05). Higher values of SMI appeared at the 12th week than at the 6th week in both groups of rats (P < 0.05). CONCLUSIONS During the healing of fracture, the BMD of the newly-formed bone reaches its peak value at the 6th week after fracture, which is earlier than the occurrence of peak biomechanical strength, area of type I collagen, and SMI. Although the collagen and aggravation of mineral determine the biomechanical properties, BMD by itself may not be able to determine the biomechanical strength because of the change of the 3-D structure of the callus. The combination of collagen and microarchitecture may act as a factor that determines the biomechanical strength of callus independent of BMD.