A design of a half-mode substrate integrated waveguide (HMSIW) cavity-backed self-diplexing antenna is proposed with a two-layer structure. The top layer comprises two HMSIW based cavities, and radiating patches are etched on the top-cladding of each cavity. The radiating patches are excited by two distinct printed microstrip lines on the backside of the bottom layer by using shorting-pins. The shorting-pin excites the corresponding cavity in its dominant mode, which resonates at two different frequencies in x-band. The simulated results demonstrate that the proposed design resonates at 8.20 GHz and 10.55 GHz with an isolation of higher than -25 dB between two excitations, which helps to introduce the self-diplexing phenomenon. Also, both resonant frequencies can be tuned independently by varying the dimensions of the corresponding cavity. Moreover, HMSIW cavity-backed structure and proposed feeding technique reduce the overall size of the antenna significantly, while it maintains high gain and unidirectional radiation characteristics for both operating frequencies. © 2018, Electromagnetics Academy. All rights reserved.