Winglets can reduce effect of wingtip vortices on the wind turbine performance can be reduced by diffusing the vortices from the blade tips. Unlike non-rotating wings, winglets have not been widely investigated for moving blades of wind turbines, while there is a potential they could enable the wind turbine rotor to capture more kinetic energy from wind. There have been a number of studies on the effect of winglet parameters and configurations on the wind turbine performance, however a combined effect of winglet planform and airfoil has not been investigated in details. The present work reports on the study of the effect of winglet planform and winglet airfoil on the wind turbine performance using Computational Fluid Dynamics (CFD). The National Renewable Energy Laboratory (NREL) phase VI rotor with 10 m diameter was used as the baseline and the CFD results were validated with the available experimental data on the output power and pressure coefficients. Different designs of winglet with different heights, cant angles, planforms and airfoils have been numerically tested and optimised. The best improvement in the performance is achieved when a 15 cm rectangular winglet with the S809 airfoil and 45 degrees cant angle is used. (C) 2018 Published by Elsevier Ltd.