In this communication, a planar high-gain slot antenna backed by a rectangular substrate-integrated waveguide (SIW) cavity is presented. The proposed antenna uses a narrow rectangular SIW cavity with high length-to-width ratio. This modifies the conventional sinusoidal field distribution of the cavity modes to generate a hybrid mode field distribution with more field concentration at the center of the cavity. A closed form expression is presented which represents the proposed hybrid mode field distribution as a summation of multiple TEym10 cavity modes. A slot antenna placed at the top plate of the proposed cavity is excited by the hybrid mode. Calculation of far-field radiation pattern of the proposed antenna is presented, which exhibits its potential to produce narrower main beam along with high gain. To validate the concept, a prototype is fabricated in a thin substrate (h =0.787 mm ). The experimental result shows antenna resonance at 9.5 GHz with high gain of 9.62 dBi. © 1963-2012 IEEE.