The performance of the NACA0012 airfoil deteriorates with decreasing Reynolds number owing to the flow separation at the trailing edge. In the wake region, an unstable fluctuation with a discrete frequency, known as trailing edge noise, is observed. It has become controversial whether or not the feedback mechanism between the oscillation of the trailing edge separation and the wake oscillation is actually occurring. The flow at low Reynolds number exhibits a periodic oscillation around the trailing edge of the NACA0012 airfoil. In such a case, the structure of the velocity field changes with the phase in a complicated manner. In order to explore problems regarding unsteady aerodynamic phenomena, comprehension of the flowfield structure is necessary. In addition to the experiment and the numerical computation of the Navier-Stokes Equations (CFD), the theoretical approach from the viewpoint of stability analysis will contribute to understanding the mechanism of such problems. For the purpose of discussing the trailing edge noise, this paper studies the stability of the NACA0012 airfoil using the concept of global linear stability analysis.