An air-filled substrate integrated waveguide (AFSIW) slot array antenna based on multilayer PCB process is proposed. For comparison and demonstration purposes, 1 × 4 slot array antennas based on both AFSIW and conventional dielectric-filled SIW (DFSIW) operating at 30.5 GHz are designed, fabricated and measured. The exampled AFSIW 1 × 4 slot array antenna achieves 44 and 18% narrower measured H-plane and E-plane half-power beamwidth, respectively, and simulated radiation efficiency improved by 1.6%, resulting in an overall measured gain improvement of 3.5 dB compared with its DFSIW counterpart. This antenna is of high interest for the design of efficient arrays based on high-performance AFSIW feeding network.