A circular polarization antenna includes a substrate, a ground plane, a tuning stub, a feeding element, and a cavity structure. The substrate has a first surface and a second surface. The feeding element is disposed on the first surface of the substrate. The ground plane is disposed on the second surface of the substrate and has a hole. The tuning stub is disposed on the second surface of the substrate and connected to the edge of the hole. The cavity structure is connected to the ground plane and configured to reflect an electromagnetic wave.