In this communication, we present a novel design of a dual-band high-gain resonant cavity antenna (RCA) with single polarization. First of all, the theory of dual-frequency operation of conventional RCA with single superstrate is discussed. Next, we designed a feed system, for dual-band operation with single polarization and directional patterns, two T-shape slots in nonradiating edges of the microstrip patch are introduced, which serves as the feed system of the designed dual-band high-gain RCA. Then, a full-wave analysis is performed to extract the impedance matching and radiation patterns of feed system and the complete RCA. At the last, measurement results are provided to compare with the modeling and simulations results which agree reasonably well.