The present study considers the use of a slat near the leading edge of an airfoil to delay or completely cancel separation and thus improves the performance of small wind turbines, in addition to lowering the cost of the blade by reducing the used material. Flow around the S809 airfoil with a slat is numerically simulated using ANSYS 2019 R1 (CFD) program. The present work concentrates on two geometrical parameters namely the location of the slat relative to the base airfoil and the chord size of the slat. Relative to the total chord length of the main airfoil, a slat with a chord length 10% is located at range of distances +3.6% to +10.8% in y direction, slat located at 9% shows the best lift coefficient. To study the effect of slat size, slat sizes range from 5% to 12.5% are numerically investigated, slat size 7.5% shows the best lift coefficient and attached flow over the airfoil for a wide range of angles of attack. The improvement is achieved with the same total chord length with 12% saving of material which corresponds to 6% of the total cost of the turbine.