The development of a sensor of temperature based on optical fiber sensors is presented. The sensor will be used in an experimental hyperthermia laboratory to characterize applicators and temperature distribution in phantoms exposed to controlled electromagnetic radiations. The temperature sensor is built with a glass optical fiber, in which part of their cladding was replaced by an oil. The variation of the oil refractive index with temperature modulates the optical power transmitted along the fiber, then the probe works based on the principle of evanescent field sensors. As advantage, the temperature sensor is not affected by the eIectromagnetic radiations. The measurements have a repeatability error less than 0,5 YO in the range of interest (20°C to 40°C). The sensors have a low thermal time constant (1,9 s), then, the time required to perform a complete mapping in a plane of the phantom is reduced.