A relatively simple two-dimensional subsonic unsteady aerodynamic theory has been developed to take into account approximately the effect of flow separation on unsteady airloads for high incidence angles of thin airfoil sections. The theory is an extension of the classical linear potential flow theory for thin airfoils with proper boundary conditions applied to the flow separation region along the airfoil surface. The mixed boundary value problem for the lifting unsteady aerodynamics is translated into two singular integral equations that are solved by using the collocation method. Very encouraging correlation has been obtained between the calculated and experimental pitch damping values for a thin airfoil section typical of propeller blade at large angles of attack.