A D-band SIW slot antenna was proposed in this paper. The feeding network consists of a CPWG input, a CPWG-SIW transmission with the wideband and low-loss characteristics from the quasi-TEM mode to quasi-TE10 mode and a rectangular slot with functions of vertical transition, transmission and power-dividing. High-gain radiation was achieved at the SIW slot antenna with good isolation between radiation field and the feeding network. As shown in the simulation results, the bandwidth of 1×4 antenna array achieved an E-plane beam-width of 24° and an H-plane beam-width of 10° at 120 GHz. The gain is more than 18.29 dBi and the VSWR is less than 2.0 with the size of 18.73 mm×5.83 mm×0.55 mm.