An organic solar cell and a method of fabricating the same are provided. The organic solar cell includes a first electrode and a second electrode. An organic active layer is disposed between the first electrode and the second electrode. The organic active layer includes an concave-convex pattern in a surface adjacent to the second electrode. The concave-convex pattern may be formed by contacting an elastomer stamp and a top surface of the organic active layer. The elastomer stamp may be formed by molding using a template having a surface relief grating (SRG). The template may include a photoisomerization polymer layer, and the surface relief grating may be formed by irradiating interference light onto the photoisomerization polymer layer. The surface relief grating may be a blazed diffraction grating.