Abstract: In this paper, a high-performance waveguide diplexer for X-band antenna system is proposed. The diplexer covers Rx-band with 7.25 GHz~7.75 GHz and Tx-band with 7.9 GHz~8.4 GHz. The diplexer is composed of two filters and a T-junction. The filters are designed by multi-mode resonators while the Mode Matching Method is used in the design of the diplexer. The easy fabrication, less insertion loss and higher power capability are realized in the proposed diplexer. High isolation between Tx and Rx ports of the diplexer is achieved in small frequency bandgap. The diplexer is fabricated and measured, the measured results show that the diplexer works in the Rx(7.25 GHz~7.75 GHz) and Tx(7.9 GHz~8.4 GHz) bands. The isolation between the transmitting and receiving ports is better than 85 dB in both Tx-band and Rx-band. Moreover, the measured return loss of the filter is less than -20 dB for each band. The insertion loss of the proposed diplexer is less than 0.47 dB.