In the present work, flow around a horizontal axis wind turbine is investigated in co-rotating and counter-rotating configurations. Large Eddy Simulation (LES) has been employed in order to study the effect of rotation direction of the downstream turbine in a two in-line setup. The results are in good agreement with presenting experimental data in literatures. The power efficiency of the downstream turbine has increased about 4% in the counter-rotating configuration rather than co-rotating configuration with the same separation distance (3D). The power efficiency of the downstream turbine increased about 7% by decreasing the separation distance to 1D. The improved efficiency in the counter-rotating configuration is due to the flow rotation direction of the upstream turbine. The results revealed in both configurations that streamwise velocity is almost identical, while lateral velocities are changed greatly. Lateral velocities are decreased by moving in a downstream direction. Hence, the benefit of this configuration is obtained by lower separation distance. The authors suggested counter-rotating configuration for the terrain with a high density of installed wind turbines and believed that operating the wind turbines in counter-rotating configuration will not only improve the total efficiency of two in-line turbines, but will also considerably reduce the space they require. Since the oscillations of power coefficient are so high, the exact location of encountering the vortices on downstream blades should be determined to improve the power coefficient of two in line turbines. So, further studies are suggested to extract and track the vortices for future work to increase the efficiency of wind farms. (C) 2017 Elsevier Ltd. All rights reserved.