In the paper, a design of millimeter-wave circularly polarized patch antenna for series-fed array is proposed with enhanced axial-ratio bandwidth. Unlike of the traditional designs, which are often with square patch, the proposed design uses rectangular patch. Therefore, the area of patch is decreased. The proposed antenna is fed by the aperture coupling method. The rectangular patch is the main radiator, and the C-shaped aperture also participates in radiation. The aperture is added with an additional slot, so that the antenna can produce circular polarization, with an enhanced axial-ratio bandwidth. The measured impedance bandwidth of the 28 GHz single circularly polarized aperture-coupled antenna is 21 %, the 3-dB axial ratio bandwidth is 5.9 %, the gain is 2.3 dBic, and the simulated front-to-back ratio is 13 dB. The simulated impedance bandwidth of the 28 GHz series-fed four-element circularly polarized aperture-coupled antenna array is 8.4 %, the 3-dB axial ratio bandwidth is 16.6 %, and the gain is 8.9 dBic,.