A counter-rotating wind turbine having two rotors rotating in opposite directions on the same axis is proposed to improve the aerodynamic performance of a wind turbine. In order to predict the aerodynamic performance of the counter-rotating wind turbine, the inflow interference in its rear rotor by the wake of the front rotor needs to be considered because the rear rotor operates inside the wake of the front rotor. In the previous research, the rear rotor was assumed to operate inside the fully developed stream tube of the front rotor to define the inflow condition on the rear rotor. In this study, to consider simultaneously the aerodynamic interaction between the two rotors of the counter-rotating wind turbine without any assumption on the inflow velocity of the rear rotor, an aerodynamic analysis of the counter-rotating wind turbine was carried out by using a free-wake vortex lattice method. The power coefficient and the wake geometry of the counter-rotating wind turbine were compared with those of a single rotor wind turbine, and the induction factors on the rear rotor were compared with that from the BEMT, assuming that the rear rotor operated inside the fully developed stream tube of the front rotor. (C) 2012 Elsevier Ltd. All rights reserved.