In this paper, a dual-frequency substrate integrated waveguide (SIW) longitudinal slot array antenna has been implemented with the combination of two 1×4 slot array etched on the top metallic surface of the line. The two arrays resonate at two distinctive frequencies (8.6 GHz & 10.8 GHz) within X band (8 - 12 GHz) with good radiation pattern at both frequencies along with a gain of 7.7 dB and 9 dB respectively. The two arrays have been fed by a single SIW line which reduces the complexity of feeding network. The proposed design shows good potential to control radiation pattern and side lobe level independently at both frequencies. © 2013 European Microwave Association.