In this article, a design of planar cavity-backed antenna-triplexer is proposed for multi-band communication systems. Substrate integrated waveguide (SIW) technology is used to realize a planar cavity structure. The proposed antenna operates at triple frequency bands (around 5, 6.4 and 7.5 GHz) simultaneously utilizing a rectangular patch and two half-mode-like-cavity resonators on the top surface of the cavity. The cavity resonators and patch are excited by microstrip feedlines and coaxial feed, respectively. An isolation of better than 20 dB is achieved between any two input-ports which confirm the triplexing phenomenon. Finally, the proposed antenna is prototyped and experimentally verified which are agreed well with simulated ones. Additionally, the antenna has the advantages of compacted-size, low-profile with planar integrability. © 2018 IEICE