A low-profile circularly polarized (CP) patch antenna with improved axial-ratio (AR) bandwidth and high gain is proposed in this letter. The antenna consists of a pin-loaded patch and a probe-fed ring resonator beneath the patch radiator. On the one hand, the radiative patch dominates the radiation, and the loading of shorting pins facilitates its high gain due to enlarged radiating area at resonance. On the other hand, the ring resonator with perturbation produces two additional split degenerate modes, which not only widen the bandwidth of magnitude, but also generate the required quadrature phase shift for CP radiation. The working mechanism is presented with an equivalent circuit model, allowing us to easily demonstrate how the ring resonator and patch radiator work together to effectively widen AR bandwidth. With the proposed method, stable high gain of about 11 dBic has been realized over an AR bandwidth of 6% for a single-fed CP patch antenna with profile of 0.05λ at 2.33 GHz.