A high-gain, low-profile electromagnetic band-gap (EBG) resonator antenna based on a thin frequency-selective-surface (FSS) superstrate is presented. The superstrate thickness has been reduced from 0.25λg in the previous design to about 0.06λg without significantly affecting the antenna performance. The FSS superstate is made out of standard low-cost FR4 material. The high-gain performance is achieved by optimizing the metallic patterns printed on the two surfaces of the superstrate so that a high-Q resonant cavity is obtained. A prototype demonstrates that an EBG resonator antenna made out of a 0.8 mm thick optimized superstrate can provide a gain as high as over 21 dB at the operating frequency of 12 GHz.