We present and characterize theoretically and experimentally a photorefractive velocimeter. This device, based on two wave mixing in a rapid photorefractive crystal, measures the instantaneous velocity of a vibrating target. It is particularly adapted to the measurement of high amplitude (as high as some mm) low frequency (until some kHz) vibrations. Instantaneous velocity as high as 25mm.s-1 are expected to be measured with common photorefractive semiconductors and CW lasers