Abstract: A phase-gradient metasurface realized by the compact Jerusalem cross elements is proposed in this paper, which is loaded on the surface of the slot array antenna. By using its characteristics of converting propagating wave(PW) into surface wave(SW) and anomalous reflection, a remarkable monostatic radar cross section reduction(RCSR) can be obtained. Compared with the reference antenna, the monostatic RCS of the proposed antenna is reduced over a frequency range of 6-18 GHz for both normal and oblique incident waves with TE and TM polarizations, and the RCS of the proposed antenna can be reduced as much as 20 dB at around 9.5 GHz. Compared with the traditional RCSR methods, this technique can reduce both in-band and out-band RCSs of the slot array antenna, whose radiation performance is preserved at the same time.