A side weir is one type of overflow structure that is installed in the side of a channel to evacuate the excessive flow laterally. A simple way to improve the hydraulic performance of this structure is the application of guide vanes. In this study, discharge coefficient and characteristics of flow over rectangular sharp-crested side weirs in the presence of guide vanes were experimentally investigated. In addition, the behavior of the flow in the absence of vanes was considered for comparison. Analyzing the measured 3D velocity distribution data indicated that in both presence and absence of guide vanes, the maximum and minimum of the longitudinal velocity occurred at the beginning and end of the side weir, respectively. In comparison with the no-vane case, the presence of guide vanes significantly decreased the longitudinal velocity. In both cases, the direction of flow near the water surface was downward, and with approaching the weir crest, the flow direction was reversed and the upward flow was generated. The presence of guide vanes increased the magnitude of downward velocity and adversely reduced the magnitude of upward velocity over the crest. Furthermore, the maximum of the lateral velocity in both cases was observed at the end of the side weir. But the presence of guide vanes increased the lateral velocity, thereby resulting in more discharge over the side structure. Data indicaes that using guide vanes cause to increase of discharge coefficient about to 30%.