A Resonant Cavity Antenna(RCA) with a non-uniform metamaterial superstrate is proposed. A rectangular patch antenna, working at 10 GHz, is placed in a cavity as a primary source. A superstrate printed with square metal patches is placed in front of the antenna in order to increase the gain. Different from normal uniform metamaterial superstrate, the proposed structure consists of 9?9 units with different sizes. Simulation results demonstrate that the directivity of non-uniform metamaterial superstrate RCA increases by 1.2 dB(from 20.3 dBi to 21.5 dBi), and the side lobe is suppressed(the main-to-sidelobe ratio decreases by 5 dB), while the other properties like antenna resonant frequency and impedance bandwidth remain unchanged almost.