Objective To investigate the changes of bone mineral density (BMD) and biomechanical properties of the femur after ipsilateral tibia fracture in rats, and its relationship with the tibia fracture union. Methods Forty 3-month-old female SD rats were divided into 2 groups randomly and evenly: the surgery group and the control group. The surgery group received middle shaft fracture and fixation surgery on the right tibia. BMD of the right femur was measured both before the surgery and at 2nd, 4th, 6th, 8th, 10th, 12th week after the surgery, respectively. Ten rats in the surgery group and ten rats in the control group were executed at 6th and 12th week after the surgery, respectively, to test the biomechanical properties of their right tibia and femur. Results The radiological union rate and mechanical union rate of the tibia were 50% and 70%, respectively, at 6th week after the surgery, and those at 12th week after the surgery were both 100%. At 2nd, 4th, 6th, 8th week after the surgery, BMD of the right femur in the surgery group was significantly lower than that in the control group (P<0.05). Compared with the control group, there was no statistical difference in BMD of the right femur at 10th, 12th week after the surgery. Biomechanical properties of the right tibia and femur at 6th week after the surgery showed obviously worse than those at 12th week after the surgery (P<0.05). Analysis showed that the fracture union was highly related with mechanical properties of the tibia (P<0.01), which was also highly related with BMD and mechanical properties of the femur (P<0.001). Conclusions In the early stage of tibia fracture, disuse osteoporosis might occur due to the decrease of BMD and worse mechanical properties in the ipsilateral femur, but as the tibia fracture united, BMD and biomechanical properties of the femur became normal again.