A large frequency interval dual-band high-gain slot array antenna based on SIW (substrate integrated waveguide) for simultaneous microwave and millimeter wave application is proposed in this paper. The designed antenna consists of 2 arrays working on 9.86 GHz ~ 10.2 GHz and 25.9 GHz ~ 39.53 GHz, which can meet demand on X- and Ka-band, respectively. Two substrates sandwiched between three layers of metal are employed, and all slots are located on the second metal layer.