The working principles of Interferometric Rayleigh scattering velocimetry using a Fabry-perot interferometer is presented. An interferometric Rayleigh scattering velocimetry apparatus consisted of a high resolution F-P interferometer, an injection-seeded Nd∶YAG laser and an intensified CCD was developed. In theory, the measurement uncertainty of velocity is 10 m/s, the recommendations for system improvements were given. Non-intrusive measurement of velocity in unseeded gas flow is performed to demonstrate this technique, flow velocity of 366 m/s was obtained in a supersonic free air jet flow.