Abstract: In this paper, a fiber Bragg grating (FBG) based temperature sensor is presented and its temperature measurement mechanism is explained. A folded MachZehnder interferometer composed of two identical FBGs is employed in the sensing system. One of the gratings is used as a reference arm (local oscillator) and another as a sensing arm. The technology of heterodyne detection is utilized to measure the physical quantity of ambient temperature，since the wavelength of the FBG variates with the temperature .The heterodyne detection is used to detect the output signal frequency difference between the reference arm and sensing arm, which is caused by the temperature variation. The dynamic range and sensitivity of the system are analyzed and presented.