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We propose an ultracompact microcavity fiber Fabry-Pérot interferometer (MCFFPI) based on an ultrathin film of gold (Au) embedded in a single-mode fiber endface. The embedded Au thin film splits the input light into two paths for interference. The interference fringes of the proposed configuration with different cavity lengths are observed experimentally, and we verify that the interference patterns are from a combination of the Au-embedded reflector and Fresnel reflection of fiber endface. The MCFFPIs are further applied to measure the external refractive index (RI) of surrounding and for high temperature (T) sensing. Experimental results show that an excellent sensitivity of the RI sensing and a good linearity response of high temperature measurement are achieved.