The design optimization of the flap position was performed in order to improve the aerodynamic performance during the take-off and landing processes of aircraft. For the optimization, the existing airfoils of the main wing and flap are selected as the baseline model and the lift coefficients (cl) according to angle of attacks (AOA) were calculated with the change of the position of flap airfoil. The objective function was defined as the maximization of the average value of the lift coefficient for removing the flow separation at low AOA. From these processes, the optimal position of flap was obtained at which cl increases linearly comparing with that of the baseline model.