Nowadays, the RF system requires a small size for compact package passive and active devices, include antenna devices. This paper proposes the half mode substrate integrated waveguide (HMSIW) structure combined with the antenna slot known as HMSIW cavity backed slot antenna (CBSA). It is achieved by bisecting the conventional SIW along the fictitious magnetic wall. By using parametric studies for the slot position and length in the ground layer, the frequency resonance shift to the lower frequency and it can be applied for antenna size reduction. The simulation results give return loss at 18.55 dB with resonance frequency of 2.43 GHz, fractional bandwidth of 1.65 % and gain of 4.12 dB where it is suitable for industrial scientific and medical (ISM) radio bands. The measurement of fabrication antenna gives resonance frequency at 2.45 GHz with fractional bandwidth of 1.22 % and gain of 3.64 dBi. Therefore, it is shown that the proposed antenna can effectively reduce its size without sacrificing its performance.