Abstract: A novel high impedance surface (HIS) structure with two inphase reflection band gaps is presented to reduce the inband radar cross section (RCS) of microstrip antenna. The HIS structure is constructed by etching the slots on the patch of a mushroomlike EBG structure, and the proposed HIS structure is added around the dualband antenna. Compared to the reference antenna, the simulated and measured results show that the radiation performance of the antenna is unchanged when the HIS structure is used, and at the same time, at two operating frequencies, the simulated RCS of the antenna with HIS structure is reduced 5.5dB and 6.5dB, respectively. 