In this letter, a new substrate-integrated waveguide (SIW) H-plane horn antenna introducing a slot and a short-terminated SIW has been proposed on a thin substrate with 0.02λ0. The slot represented by a capacitor in parallel with a radiation resistance and the short-terminated SIW modeled as an inductor play key roles in impedance matching between both substrate and free space at the aperture. The designed SIW H-plane horn antenna provided a 10 dB bandwidth of 27.49 GHz to 28.54 GHz. The peak antenna gain was achieved as 10 dBi at 27.925 GHz.