An intrinsic Fabry-Perot interferometric fiber sensor is successfully fabricated by normal single mode fiber and multimode fiber with different outer diameters. Its temperature and pressure response performance is investigated. The pressure responding theoretical model is got by materials mechanics theory and photoelastic theory. Considering the strain direction caused by Poisson effect, the multimode fiber with small diameter is used to improve the pressure sensitivity. In experiments, the diameter of multimode fiber is 120 mm after the fiber is corroded by hydrofluoric acid. The temperature responding experiment is done by this sensor with varied temperature from 20 ℃ to 500 ℃, with 1 mm cavity length. The experiment results show the sensor have a good temperature linear response and repeatability. The temperature sensitivity is 25 nm/℃ . The pressure experiment results are also got at 20 ℃ temperature. The experiment results show the sensor also has a good pressure linear response. The pressure sensitivity is 4.7 nm/MPa in this experiment. This kind of optic fiber sensor can be well responded by temperature and pressure, and with the potential of multiplexing sensors.