In this study, longitudinal dynamic derivatives of an airfoil of the type NACA 6-series, oscillating in pitching and plunging motions were calculated using variation of pitching moment coefficients with angle of attack in various conditions, based on wind tunnel data. Various parameters of the tests were mean angle of attack, reduced frequency and amplitude of oscillation. To calculate the longitudinal dynamic derivatives in harmonic oscillations, the Taylor's series and integral of Fourier were used. Both the methods had the same results and could be extended to each flight vehicles. The effect of parameters on variation of longitudinal oscillatory derivatives was investigated, in three different regions of oscillation: before, over and post stall conditions. The results showed that variation of the longitudinal oscillatory coefficients with angle of attack is different in the pre-stall and over stall conditions with respect to post-stall region. The effect of reduced frequency on stability of the motion is different for two types of oscillations. Increasing the reduced frequency resulted in reducing the stability of plunging motion, but has a little effect on the stability of pitching motion.