Passive Flow Control (PFC) technique has been established as an effective approach in the mitigation/elimination of flow separation phenomenon to enhance the aerodynamic performance of the wind turbine blades. The paper sheds light on the working mechanism of slot and Vortex Generator (VG) in the transitional boundary layer which has received far less attention from researchers. Also, the idea of the combination of two PFC techniques including the slot and the VG treatments on the wind turbine blade has been tested. The lift to drag coefficient (CL/CD) improves up to 112% at incidence angle of 16.3° using VG in comparison to uncontrolled case; however, this coefficient deteriorates down to 12.3% on average for pre-stall angles. The results revealed that the slot exit jet at the aft portion of chord length (70% C) improves the lift to drag coefficient (CL/CD) by 91% at the post-stall condition, a reduction of aerodynamic characteristics over the airfoil profile is detected for the lower inflow angles. The combined PFC method shows better performance than the slotted profile in terms of lift coefficient in most operating angles, but still has lower values than the controlled case with VG.