We propose a novel design to reduce the radar cross section (RCS) and enhance the gain of a patch antenna by using partially reflecting surface (PRS). The PRS consists of two layers of metallic patterns on both sides of a dielectric slab. The metallic pattern on the bottom side is utilized to construct Fabry-Perot resonance cavity with ground plane of a patch antenna, while it is designed to absorb most of the incident wave on the top side. The PRS can enable the patch antenna to simultaneously achieve high gain and low RCS. Measurement results show that the antenna gain is enhanced by about 6.5 dB at 11.5 GHz, and its RCS is dramatically reduced in a broad frequency range from 6 to 14 GHz, compared with the traditional patch antenna without the PRS.