A Fabry-Perot cavity (FPC) antenna producing both high-gain and circularly-polarized (CP) behavior is proposed. To increase antenna gain and obtain CP characteristics, a superstrate composed of square patches with a pair of truncated corners is placed above the linearly polarized patch antenna with an approximately half-wavelength distance from the ground plane at the operating frequency. The proposed antenna has the advantages of high gain, a simple design, and an excellent boresight axial ratio over the operating frequency bandwidth. Moreover, used in an FPC antenna, the proposed superstrate converts a linear polarization produced by a patch antenna into a circular polarization. In addition, the cavity antenna produces left-hand circular-polarization and right-hand circular-polarization when a patch antenna inside the cavity generates x-direction and y-direction polarization, respectively. The measured and simulated results verify the performance of the antenna.