In this paper, effect of the bed roughness on the flow pattern in a wall jet is studied experimentally. Channel that experiments has been carried out included glass walls and metal floors with 0.57 meters width, 1 meter height and 10 meters length. Jet enters horizontally to the center of channel through a pipe with 20 millimeters diameter. At first the experiments were done using three dimensional velocimeter -ADV- for measuring of velocity components at three directions. Then the collected data were processed and used for drawing flow pattern along the measuring domain. It was found that the zone of the potential core of jet occurs at the distance of about five times of the jet diameter. By increasing the bed roughness, the lateral and vertical components of velocity of jet increases. By increasing the bed roughness, the growth rate of longitudinal scale decreases and the decay rate of velocity of jet increases.