Abstract: Half-Mode substrate integrated waveguide(HMSIW) was proposed in recent years as a kind of high-performance planar microwave transmission structure. With the advantages of low transmission loss, high power handing capacity and high integration ability. A number of high-performance components have been developed using this technique. Based on the HMSIW technology and the printed circuit board(PCB) process, a novel three aperture directional coupler is designed by etching I-shaped slots on the metallic surface of coupling aperture and verified with experiments. With all advantages of the substrate integrated waveguide(SIW), the proposed coupler is of 50% reduction in size and obtains 1GHz bandwidth in operating frequency. The measured results are in agreement with the simulated results by using Ansoft HFSS 10.0, which validates the concept of the HMSIW three apertures directional coupler and its design method.