In this letter, a novel dumbbell-shaped slot along with thin substrate integrated waveguide (SIW) cavity backing (height < 0.036λ0) is used to design dual-frequency slot antenna. The proposed design exhibits unidirectional radiation characteristics, high gain, high front to back ratio (FTBR) at each resonant frequency while maintaining low profile, planar configuration. The unique slot shape helps to introduce complex current distribution at different frequencies that results in simultaneous excitation of hybrid mode at higher frequency along with conventional TE120 mode in the cavity. Both conventional mode and the hybrid mode helps the modified slot to radiate at the corresponding resonant frequencies resulting in compact, dual-frequency antenna. A fabricated prototype is also presented which resonates at 9.5 GHz and 13.85 GHz with impedance bandwidth more than 1.5% at both resonant frequencies and gain of 4.8 dBi and 3.74 dBi respectively. The front-to-back ratio of the antenna are above 10 dB at both operating frequencies. © 2014 IEEE.