A simple fiber optic temperature sensor with a micro Fabry-Perot cavity constructed with a thin film coated on a fiber end is presented. Its operation principle is based on the temperature-dependent wavelength shift of the interference fringes formed by Fresnel reflections from both interfaces of the thin film. An analytical formula is derived from the theory of two-beam interference. Experimental temperature data agree with the theoretical result. A temperature resolution of 0.1 K within the range from 20 to 100 °C with good stability can be achieved. The proposed temperature sensor has a simple, solid, and compact structure.