ABSTRACT: In this study, a fiber optic temperature sensor was developed using an aqueous solution of sodium chloride (NaCl solution) and an Optical Time-Domain Reflectometer (OTDR) to measure temperature at a long distance. By changing temperature, the refractive index of the NaCl solution is varied and then Fresnel reflection arising at the interface between the distal end of optical fiber and the NaCl solution is also changed. Therefore, we measured the optical power of the light reflected from the sensing probe according to the temperature of water and also obtained the relationship between the temperature of water and the optical power.