Abstarct: Spillways are the most important part of dams that have fundamental role on dam safety and flood passing capacity. The ability of spillways on passing reservoirs surplus water has a great effect on increasing the reservoir capacity. Due to growing water demand, design of new spillways with high performance is critical. The piano key weir is one of the spillways that have good performance on passing water. Different physical models and experimental studies is done for weir. Due to shape complexity of this weir, there is no accurate relationship to design this weir. The numerical models of this weir was implemented by computational fluid dynamic software “FLOW-3D” and the performance of different geometrical parameters were examined by discharge coefficients. In this study, design pattern, presented by Lemperier in 2009, was applied to improve the performance of the weir. Also installing the parapet wall on top of weir were examined to increase the efficiency of the weir. Both of using new pattern and installing of parapet wall had positive effect on the efficiency of the weir. The modeling result showed that using Lemperier pattern can increase the discharge coefficient. This increase was over 40% in high heads. Also studying the parapet wall in different specific case showed the different effect on different parts of weir. Using parapet wall in the outlet keys had good performance, although using it in inlet key had negative effect. With regards to this result a new idea, named inclined wall, was examined for parapet wall on the weir. We found positive result and it can be developed in future.