In reference to the dynamic stall problem of helicopter rotor blades, a semi-empirical theory is presented for predicting unsteady lift and moment on an oscillating two-dimensional airfoil, in the presence of dynamic stall, from static airfoil data. Based on the concept of equivalent angle of attack, the time delay of dynamic stall is considered and the linear potential flow theory is used for the calculation of wake vortices and their effects on the equivalent angle of attack. The theory can be applied to the case of nonsinusoidal airfoil oscillation as well as sinusoidal one since it utilizes the FOURIER expansion technique. It is shown in numerical examples that the theoretical results are in good agreement with available dynamic experimental data.