Abstract: The paper proposed a cavity-coupled vertical transition structure working at 110 GHz. Two mode-transition units were fabricated at ends of a vertical metal cavity symmetrically, acting as two excitation ports of a waveguide. The proposed mode-transition unit was realized on a 50-μm thick quartz-substrate with via holes and double-side patterned. In this way, the vertical transition structure presented a low insertion loss at terahertz frequency. Good agreement between simulated and measured results was obtained. The simulated S₂₁ of the mode-transition unit was −0.7 dB, the measured S₂₁ was less than −1.3 dB. The bandwidth from 105 GHz to 116 GHz was obtained for reflection level lower than −10 dB.