A millimeter-wave broadband circularly polarized antenna sub-array is proposed. This sub-array consists of four units. The overall size for the sub-array is $10.2 \mathrm{~mm} \times 10.2 \mathrm{~mm}$ which is approximately equal to $\lambda_{0} \times \lambda_{0}$, where $\lambda_{0}$ is the central wavelength, and the spacing between the four units is about 0.5 $\lambda_{0}$. Specifically, by using two-point feeding, multi-layer patch, parasitic loading and sequential rotation feeding technique, the current sub-array presents excellent characteristics in bandwidth, gain, as well as axial ratio. The operating band is from 26.85 GHz to 32.06GHz with fractional bandwidth (FBW) is 17.7%. The axial ratio is less than $0.2 \mathrm{~dB}$ in the operating band, showing a strong potential for aerospace, radar, and 5G communication systems.