Objective To predict the micro-level energy release rate in the rat femoral cortical bone and investigate the variation in the micro-level energy release rate with age. Methods Based on previous experimental data and numerical simulation of fracture modes for cortical bone, load-displacement curves and fracture modes measured by simulation and experiment were compared, and the micro-level energy release rates of rat femoral cortical bone at different months were predicted by back-calculation. Results It was predicted that the micro-level energy release rate of rat femoral cortical bone at 1-, 3-, 5-, 7-, 9-, 11-, and 15-month age was 0.08–0.12, 0.12–0.14, 0.15–0.19, 0.25–0.28, 0.23–0.25, 0.19–0.22, and 0.13–0.16 N/mm, respectively. Conclusions The decrease in the microlevel energy release rate with increasing age led to a decreasing failure load, indicating that the microlevel energy release rate is one of the main factors determining fracture occurrence; however, no significant decrease was observed at the time of fracture, indicating that the microlevel energy release rate was not linearly proportional to the fracture time. These results can help explain the mechanism of cortical bone fractures at the clinical level.