This paper investigates the effect of profile-modifications on a NACA-0015 aerofoil used in VAWTs (vertical axis wind turbines). The profile-modifications being investigated consist of a combination of inward semi-circular dimple and Gurney flap at the lower surface of the NACA-0015 aerofoil. Rather than maximize the lift-coefficient or the ratio of the lift to drag coefficients, this paper choose to maximize the average (or effective) torque of the VAWT as this is a much better measure of the power produced. A fully automated optimization using RSA (Response Surface Approximation) is utilized here to maximize the average torque produced by the wind turbine blade. The data-set used in the optimization is generated using CFD (computational fluid dynamics) simulations. In order to ensure reliability, the computational domain and the turbulence model used in the CFD simulations are validated against previous experimental results. The optimized shape of the modified aerofoil is shown to improve the aerodynamics of the wind turbine blade. (C) 2014 Elsevier Ltd. All rights reserved.