A fiber sensor system was developed to measure high temperatures. The system consists of a diode laser as the light source, three optical couplers, two sensing fiber ends, and two photodetectors. The measurement is based on the relative Fresnel reflection intensity and the thermo-optic effect of the fiber. The application of this relative technique can operatively eliminate the errors resulting from light source fluctuations and the effect of the environment. The stability of the system for long-duration measurements can also be effectively improved.