Rectangular dielectric loaded substrate integrated waveguide (SIW) H-plane horn antenna, operating at 26 GHz has been proposed in this paper. For the simulations, a substrate of thickness 3.2mm and a dielectric material of Poly Tetra Fluoro Ethylene (PTFE) has been used. The size of antenna was reduced and a dielectric slab was loaded in front of the horn. This resulted in the increase of gain and also narrow beamwidths in the E- plane and H- plane. A comparison between two horn antennas, one without dielectric loading and one with dielectric loading has been presented in this paper