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During aircraft development process, aeroelasticity design must be considered. As the way to acquire aeroelasticity characteristics, eigenvalue approach of airplane structural dynamics is usually applied. This approach can tell us the transition of eigen frequency and damping rate of structural vibration modes as the flow dynamic pressure changes. The coupling of structural eigen modes during flutter occurrence can be captured by this method. On the other hand, the influence of one eigen mode on other modes can’t be understood by this method. In order to know this influence, the ‘Step by Step (SBS)' method is introduced in this study: SBS is originally developed in the civil Engineering for the flutter analyses of suspension bridges. In this study, two-dimensional airfoil vibration model is considered. In the formulation, the linear unsteady aerodynamics are applied to the damped oscillations; one freedom can describe another one by use of the gain ratio and the phase-difference angle therebetween. Although the original SBS assumes harmonic oscillations for aerodynamic forces, in this study, damped harmonic aerodynamic oscillations caused by damped structural oscillations are considered: then the authors call their method as ‘Modified SBS (MSBS)’ method. The computed results by MSBS are shown with comparisons among those from the current eigenvalue approaches.