This paper presents a computational investigation on the effects of Gurney flaps on the aerodynamic performance of a horizontal axis wind turbine, which is part of the EU FP7 AVATAR project. The research investigates two configurations of Gurney flaps applied at the inboard part of the blade (r/R=0.30∼0.46) at 85% chord location on the pressure surface. The computational method applied in the investigation solves the Reynold-Averaged Navier-Stokes (RANS) equations with multiple reference frame (MRF) approach, which models the rotating turbulent flow over the wind turbine rotor. Numerical simulations are performed for the wind turbine rotor with and without Gurney flap at the tip speed ratios λ=4.59 and 6.35. Comparison of the numerical results with experimental measurements shows that the deployment of Gurney flaps effectively increases the power coefficients of the rotor by 21% at λ=6.35. Gurney flaps have a considerable 3D effect on spanwise thrust and torque coefficients distribution. The performance of two Gurney flaps configurations is compared. It is shown that the larger Gurney flap reduces the effect on the power generated due to protruding out of the local boundary layer of the flow. The numerical results are in good agreement with the experimental results in terms of total thrust and power within 14.1% difference, and complement the experimental database.