In this paper, a substrate integrated waveguide (SIW)-based wideband circularly polarized (CP) 4×4 patch array antenna operating in 45 GHz band is proposed. Firstly, a series-fed 1×4 patch subarray is designed. Sequential rotation (SR) feeding technology is adopted to extend axial-ratio (AR) bandwidth. Then, a 4×4 patch array antenna using four 1×4 patch subarrays is presented. The anti-phase feeding network is used to obtain stable CP radiation patterns among wide frequency band. The proposed antenna was fabricated and measured. -10-dB|S11| bandwidth of 27.6% (36.4-51.2 GHz), 3-dB AR bandwidth of 11.6% (41.6-47.6 GHz) and the peak gain of 17.1 dBic are achieved in the vicinity of 45 GHz.