Leading edge slat is a device that is installed upstream of the main airfoil to control the flow over the airfoil passively or actively in order to improve its aerodynamic performance. The present study aims to investigate the effect of using leading edge slat on controlling the flow separation over the airfoil and hence the overall performance of the airfoil for small HAWT. Ansys 2019 software is used to numerically simulate the flow over S809 airfoil. According to NREL, S809 airfoil is a good choice for small HAWT. The current study considers the effect of angle of attack of the leading edge as a geometrical parameter which has an obvious effect on its performance. Based on previous study by the authors, slat of chord length of 15mm, is located at +18mm distance in y direction relative to the base airfoil chord which is 185mm, is studied while varying its installation angle β in range of -10⁰ to +10⁰. Slat angle of +5⁰ has the best effect on the performance of the airfoil. It moves the point of flow separation forward all the way to the trailing edge of the base airfoil. As a result of improving the flow behavior, lift coefficient is increased by 40% at AoA=17.2⁰ at the same angle relative to the condition without slat.