The side weirs Including hydraulic structures that are used in flood control, urban sewage disposal networks, drainage channels. Today, with the advancement of computer science, the use of numerical models has increased dramatically because of less cost and time spent Compared to laboratory models. In the present study, using a numerical model, Investigates the flow pattern on the side weir in convergent rectangular channel, using the turbulence model RNG K- Ԑ In order to simulating the turbulence of the flow, and volume of fluid VOF method for predicting changes of flow free surface. One of the advantages of using this type of lateral overflows is that after drainage, it is no longer necessary to continue the downstream channel with the same previous width and as a result this is economically cost effective. Comparison of longitudinal velocity changes and water level profiles at different sections, and the flow discharge of side weir and main channel, with experimental results, indicates the numerical model capability to simulating a flow pattern over this type of side weir. In this paper, the effect of side weir length, on the profile and flow pattern along the side weir in the main channel is investigated. Comparison of parameters in different regions of the middle section of side weirs indicates that by decreasing the side weir length, the longitudinal velocity also decreases. Also, by reducing the side weir length, the Lateral outflow decreases and the flow surface profile over the side weir is formed at a higher elevation.