Abstract: In regard of in-band and out-band radar cross section(RCS) reduction of microstrip antenna, a method for designing low RCS antenna based on combinatorial gradient metasurface is proposed, which can release the issue about bandwidth limited and design complexity of the conventional technology. The low RCS microstrip antenna is designed by distributing eight kinds of gradient supercells around radiation patch, the supercells have different gradient directions with an interval of 45°. Due to the anomalous reflection property of the gradient supercell, the scattered field is redistributed into various directions and performs like diffusion phenomenon. Therefore, both monostatic RCS and bistatic RCS can be suppressed. The simulated and measured results show that the RCS is reduced by more than 5 dB within 7.2-18.4 GHz as well as the radiation performance was maintained after loading the combinatorial gradient metasurfaces. The design approach features properties such as design convenience and ultra-wideband operation.