In this paper, a novel design of compact chipless identification tag using half-mode substrate integrated waveguide (HMSIW) cavity resonator is proposed. The proposed tag utilizes tuning of resonant frequency of HMSIW resonator using a metallic post in the cavity. The tuning post perturbs the electric field inside the cavity and as a result, the resonant frequency of the cavity shifts towards higher frequency corresponding to different locations of the tuning post. This shifting of the resonant frequency based on different tuning post location is utilized as signature of individual tag in this design. The proposed design retains constant size for all combination of identification tag which makes it suitable for practical applications. For transmitting and receiving of signal, the proposed design uses a printed dipole antenna which is fed by a small SIW section. The use of half mode SIW cavity helps to improve the compactness of the tag. The overall tag consisting of printed dipole and HMSIW resonator is realized in a single substrate. The proposed chipless tag is very much suitable for Ka band applications. © 2017 IEEE.