Abstract: A numerical analysis for S809 airfoil and NREL Phase VI blade has been made to investigate laminar/turbulence transition effect on stall performance of wind turbine airfoils and blades.The transition prediction method is Gamma-Theta model coupled with k-ω SST turbulence model.Firstly,simulations with transition prediction and full turbulence are conducted for S809 airfoil in a range of angles of attack of 0°~30°.It is showed that there is a significant impact on the airfoil’s stall performance by the laminar separation bubble near leading-edge.Subsequently,a similar numerical analysis is performed for the NREL Phase VI blade.It is found that the way of impact on stall performance of the blade is similar to that of the airfoil.For both the airfoil and blade,transition’s effect on stall performance mainly depends on the existence of the laminar separation bubble near leading-edge,which makes the deep stall come earlier.