A metamaterial absorber with high absorptivity, wide incident angle and no surface ullage layer is designed and applied to microstrip antenna to reduce its radar cross section (RCS). The results show that the absorber can exhibit an absorption of 99.9% with a thickness of 0.3 mm. Compared with the conventional microstrip antenna, the proposed antenna has an RCS reduction of more than 3 dB in the boresight direction in the working frequency band, and the largest reduction can reach 16.7 dB, the monostatic and bistatic RCS reduction are over 3 dB from -30° to +30° and -90° to +90° respectively, while the radiation performance is kept, which proves that the absorber has an excellent absorptivity and could be applied to microstrip antennas to achieve in-band stealth.