This paper presents a 220-GHz waveguide diplexer consisting of two waveguide bandpass filters, one using a conventional resonant cavity to form a Chebyshev filter, and the other using a dual-mode resonant cavity $(TE_{301}$ and TE 102 mode) at the end to introduce transmission zero (TZ) to form a Quasi-elliptic function filter. The TZ introduced at the upper stopband of the filter improve the out-of-band rejection and the port-port isolation. Simulation results show that the passbands of the two bandpass filters are 214.1-219.2 GHz and 220.8-225.0 GHz. The in-band insertion loss is less than 0.86 dB and 0.75 dB, respectively, and the return loss at the common port is better than 20 dB. In addition, the port-port isolation is better than 39 dB due to the presence of TZ.