This paper presents the parameter influence of upstream deflector on the performance of vertical axis wind turbines (VAWTs). For this purpose, two counter-rotating straight-bladed VAWTs and the upstream deflector have been simulated via three-dimensional (3D) computational fluid dynamics (CFD), which is validated against experimental data. Parameter Influence of upstream deflector on VAWTs output power is investigated in this study. From the CFD results, it is observed that when the turbine is positioned outside the near-wake region, the aerodynamic power can increase significantly.) This research indicates that performance of VAWTs can be greatly improved by properly choosing parameters of the deflector. (C) 2017 Elsevier Ltd. All rights reserved.