A fiber-optic intrinsic Fabry-Perot (F-P) interferometric sensor was inscribed in a standard single mode fiber by inducing refractive index change in the core. It was written directly by using a femtosecond laser. The reflection spectrum of the fiber-optic F-P interferometric sensor was measured and was investigated as a temperature sensor. The sensitivity of the sensor is 9.2 pm/°C in the temperature range of -20~80°C.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.