The circularly polarized post-wall waveguide array is designed and fabricated for the first time. The aperture illumination and the characteristics of the antenna are investigated by using analysis/design method developed for oversized rectangular waveguide slot arrays. The uniformity as well as the polarization is perturbed along the periphery of the aperture; the narrow walls are identified to be the dominant factors for degrading the amplitude uniformity and the polarization purity in the aperture illumination. The peak gain of 29.3dBi with 39.7% efficiency is realized at 77GHz band for the antenna with about 5cm square aperture.