Objective To study the change of tissue oxygen index (TOI) by non-invasive near infrared spectroscopy (NIRS) and to investigate the blood flow variety of fibula flaps after operation. Methods Thirty-six patients who accepted fibula flap reconstruction were chosen as subjects. Authors measured the TOI of the fibula flaps and the control side every four hours in the first twenty-four hours, and measured these positions with the intermittence of twelve hours from second to eighth day after operation. Results Thirty-five flaps were successful and one failed. The TOI of fibula flaps shortly after operation was significantly lower than that before the operation(P <0. 05). In the successful cases the TOI of fibula flaps and the control sides was fluctuated from 50. 0% to 72. 0%. The TOI of fibula flaps was significantly lower than that of the control sides within 144 hours after operation(P <0. 05). After 144 hours the TOI was equal to that of the control side. TOI of the failed ease decreased dramatically. Conclusions NIRS can reliably indicate the change of TOI in buried flaps and detect ischemia at the early stage. The TOI of the fibula flaps depress at the early stage and returns to normal at 144 hours after operation.