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A novel high-performance planar diplexer is developed based on the substrate integrated waveguide (SIW) with complementary split-ring resonators (CSRRs) etched on the waveguide surface. The proposed diplexer is operated below the characteristic cutoff frequency of the waveguide. The filtering response of the SIW-CSRR combined structure is investigated first. The diplexer is then implemented based on two cascaded two-pole bandpass filters. Equivalent circuit, simulated results and experimental verification are all provided. This diplexer shows advantages in terms of the compact size, low loss, high isolation, easy fabrication and integration with other circuits.