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This communication presents the design of a wideband circularly polarized (CP) antenna array based on sequential rotation (SR) technique that is excited by a substrate integrated coaxial line (SICL) with mature and stable processing technology for millimeter-wave (mm-wave) applications. Initially, the influence of the amplitude and phase errors of the feeding network on the axial ratio of the CP array is analyzed, and the detailed design scheme of the wideband CP array based on SR technology is summarized, which can be used to design SR CP array with wide bandwidth and high polarization purity. Then, an SICL-fed $4 \times 4$ CP array working in the mm-wave band that is composed of two substrate layers and a bonding film layer is designed. The radiation element is processed with a larger chamfering, which can realize elliptically polarized radiation in a broadband range, and the impedance bandwidth is significantly expanded relative to the working state of linearly polarized and CP, which has a significant impact on the final array working bandwidth enhancement. The feeding network and radiation elements are directly connected in the same plane. Finally, the proposed antenna is processed and measured to verify the theory and design scheme in this communication. Over the frequency range of 25.0–34.0 GHz, the measured 1.5 dB axial ratio bandwidth of the fabricated prototype is 30.5%, and the return losses are less than −10 dB. The maximum measured peak gain is 16.5 dBi. [ABSTRACT FROM AUTHOR]