Abstract: Due to low atmospheric absorption, short wavelength, and wide available bandwidth, W-band (75-110 GHz) is widely used in radar, communications and other fields. A wideband substrate integrated waveguide (SIW) cavity-backed slot antenna is presented for W-band application in this paper. The-10 dB impedance bandwidth of 28.6% (78.93-105.24 GHz) is achieved, which covers 75% frequency range of the W-band. The antenna consists two substrate layers. A resonant radiating-element composed of an SIW cavity resonator and four radiating slots is constructed in the upper substrate. The hybrid mode of TM₁₃₀ and TM₃₁₀, the TM₃₂₀ mode, and the TM₃₃₀ mode are excited in the resonant cavity simultaneously. Along with the radiating slots, a multi-resonances characteristic is achieved to form the wideband performance. An integrated waveguide laying in the lower substrate is utilized to feed the radiating element through the upper slot, which can be easily expanded into a planar network to construct a high-gain antenna array. The antenna has wide bandwidth, low cross-polarization, low profile, easy integration with planar microwave circuits and low processing cost, which promises a good application prospect.