This study investigated the time-course of changes in bone mechanical strength in rats with spinal cord injury (SCI). Sixty-four male Wistar rats underwent spinal cord transection at the thoracic nerve. Control rats underwent a sham procedure (SHAM). Animals were sacrificed at day 1, 4, 7 and 14 after operation. The mechanical strength of the left femur and tibia was measured by the three-point bending strength test. The bones were dried, weighed and burned to ash. A specimen of right tibia was prepared and examined under a microscope. Bone mechanical strength, dry bone weight, and ash content of the femur and tibia in SCI rats were significantly lower than those in SHAM animals. Dry bone weight and ash content began to decrease from the 4th day after SCI and reached their lowest at day 7 after operation. Bone mechanical strength had reduced significantly by the 14th day. Gaps and spaces were observed in the trabecular area at the same time. After SCI, calcified cartilage decreased and the reduction of bone mass occurred rapidly. Moreover, a decline of bone mechanical strength is caused within 2 weeks. Thus, SCI led to the atrophy of bone and caused the reduction of mechanical strength at an early stage. It is thus necessary to prevent bone loss after SCI immediately.