A class of circularly polarized (CP) cavity-backed slot antennas with enhanced gain is formed up. All the proposed antennas are implemented in a single rectangular metal cavity. One slot is formed at the bottom side of this cavity as a feeding slot to excite two orthogonal resonant modes. The slot antenna is then constituted at the opposite side of the feeding slot to radiate electromagnetic wave. A two-element array is further implemented to improve the radiation gain with 3 dB enhancement without size increment of the original antenna. Finally, two CP cavity-backed slot antennas are fabricated and tested for experimental verification of the proposed design methodology. A good agreement between measurement and simulation shows the feasibility of the proposed cavity-backed slot antenna.