In this paper, a dual-frequency substrate integrated waveguide (SIW) cavity backed slot antenna has been presented. The unique slot structure includes a pair of slot stubs connected at the center of the radiating slot which helps to create an additional hybrid mode resonance at the higher frequency end. As a result, the antenna resonates at two different frequencies 8.96 GHz and 15.84 GHz with a bandwidth of 240 MHz (2.67 %) and 300 MHz (1.89 %) and gain of 4.47 dBi and 5.4 dBi respectively. The proposed design has been fabricated and measured results are also presented. The measured front-to-back ratio (FTBR) of the radiation pattern is 12 dB and 10 dB respectively and cross polarization level is 20 dB below at both frequencies in the broadside direction. © 2014 European Microwave Association.