Abstarct: Flow pattern recognition by the use of the numerical simulation may help designers to propose optimal shape for hydraulics structures to increase their performance. In this study, the shape of guide walls and forward facing step was optimized by optimality criteria method to reduce energy loss and removing the cross waves and non-uniformity of the flow. The optimization criteria was streamlining and the optimization variables defined as solid boundaries coordinates. To optimize the shape of guide walls and steps by optimality criteria method a programming code was developed to streamline the cross sections of transitional sections and called Streamlining Programming Code (SPC). SPC used C++ programming language and is able to solve the potential flows in order to obtain the streamlines coordinates and to map a selected streamline to the solid boundaries. At the first step, the initial shape is solved by the mean of potential flow using SPC. At repeating the process by mapping the streamline of interest onto the boundary, the energy loss was evaluated using OpenFOAM. Then to evaluate the effect of new geometries of guide walls on the flow pattern and rating curve of Balaroud dam spillway’s (Iran), several plans for the guide walls were simulated. After validating the results of OpenFOAM by a same experimental models in both step and guide wall, those were validated. Those results illustrated that the step's shape obtained from SPC, decreases the energy loss as much as 30% in comparison with rectangular (experimental) model and eliminate vortex over the forward facing step. Also, optimum SPC plan in guide walls had very good performance for removing cross waves and smoothly passing the flow through the approach channel and over the spillway.