A compact and simple fiber in-line modal interferometer with an embedded fiber Bragg grating (FBG) is proposed and experimentally demonstrated for simultaneous measurement of refractive index and temperature. The in-fiber modal interferometer is formed by cascading two offset-splicing joints in a section of fiber with an inscribed FBG. The interference pattern of the modal interferometer shifts with the ambient refractive index and temperature variation, while the Bragg wavelength of the FBG is only sensitive to temperature, so the proposed device can solve the cross-sensitivity of refractive index and temperature and measure dual parameters simultaneously. The refractive index and temperature sensitivities of -26.22 nm/RI and 46.5 pm/°C are obtained, respectively. The sensor is simple and compact, and shows good prospect in the application fields of chemical sensing, biochemical sensing, and environment monitoring for simultaneous measurement of refractive index and temperature.