A miniaturized, high-gain, circular polarization (CP) -reconfigurable antenna implemented by a slow-wave transmission line (SW-TL)-based annular ring is developed. First, the detailed configuration of the CP-reconfigurable antenna, together with its biasing circuit, is presented. Then, its working mechanism of CP radiation and miniaturization is comprehensively revealed by illustrating the evolution process from a conventional linearly polarized annular ring antenna. Furthermore, the detailed resonant frequency of the miniaturized SW-TL-based antenna is calculated by the slow-wave factor of the fan-shaped SW-TL. Finally, experiments were carried out to validate the design concept. Measured results, verified by the simulated ones, show that the developed CP-reconfigurable antenna exhibits a high gain of 7.83 dBic under a miniaturized annular ring radiator with radius of 0.136λ0.