The proper orthogonal decomposition (POD) is used to extract coherent structures from a turbulent leading-edge separation bubble. A two-dimensional leading-edge separation bubble is simulated using the discrete-vortex method, where a time-dependent source forcing is perturbed near the separation point. Based on the wealth of numerical data, POD is applied in a range of the forcing amplitude ($A_o$=0, 0.5, 1.0 and 1.5) and forcing frequency (0≤$f_FH/U_\infty$≤0.3). Application of POD reveals that the eigen structures are changed noticeably by local forcings. In an effort to investigate the mechanism of decreasing reattachment length ($x_R$), dynamic behaviors of the expansion coefficients and contributions of the eigenfucntions are scrutinized. As the forcing amplitude increases, the large-scale vortex structures are formed near the separation point. Furthermore, the flow becomes more organized, which results in the reduction of $x_R$. Two distinctive regimes are classified: the regime of decreaing $x_R$ and the regime of increasing $x_R$. The POD global entropy indicates that $x_R$ is closely linked to the organization of the flow structure.
