A surface micromachined Fabry-Perot cavity used as a pressure sensor has been fabricated using standard IC technology. Dielectric film stacks consisting of layers of silicon dioxide and silicon nitride were used as mirrors. Polysilicon was used as a sacrificial layer that was then removed to form an air gap cavity. The Fabry-Perot sensor was optically interrogated using a multimode optical fiber. The measured response of the sensor agrees well with theoretical simulation, which takes into account the averaging effect caused by the shape of the deflected mirror in the cavity.