The transition in the flow-periodicity behind an airfoil oscillating with prescribed si- nusoidal pitching-plunging motion has been explored by proper orthogonal decomposition (POD) technique. The high fidelity simulation of the viscous flow past a flapping airfoil has been performed by a finite volume based incompressible Navier-Stokes solver. The flow topology transitions from a periodic to chaotic dynamics as the non-dimensional plunge amplitude is increased keeping the pitch amplitude and the frequency of the periodic flap- ping motion constant throughout the simulation. The transition takes place from a periodic reverse K ́ arm ́ an vortex street to an aperiodic flow-field through wake deflection and mode switching of the vortex street as the Strouhal number is increased. In this paper, POD is opted to methodically investigate the transitional dynamics. It is an efficient technique to decompose the flow field snapshots obtained from the numerical simulations into a set of orthogonal modes of varying energy content and to identify the high energy POD modes those are sufficient to reconstruct the flow field. Interesting insights into the change of flow periodicity are obtained by comparing the dominant flow modes of disparate dynamics at different non-dimensional plunge amplitudes.