In this paper, we propose a novel electrically small dual-band reconfigurable antenna. Low- and high-frequency resonances are generated by a substrate-integrated-waveguide (SIW) cavity resonator and a complementary split-ring resonator (CSRR). The overall size is reduced because of the use of an eighth-mode SIW (EMSIW) and an evanescent-mode CSRR. The varactor-loaded CSRR enables only the high frequencies to continuously vary from 2.38GHz to 2.5GHz for channel selection of wireless local area network (WLAN), while the low frequencies are fixed at 1.575GHz for global position system (GPS), because the two resonant frequencies can be independently controlled. The proposed concept is demonstrated by the simulated and measured s-parameters and radiation patterns.