Abstract: An ultra-thin metamaterial absorber is designed and applied to reducing in-band Radar Cross Section (RCS) of waveguide slot antenna. The absorber is consisted solely of two metallic layers separated by a lossy dielectric spacer. In spite of its very thin structure, almost/175, the absorber achieves a maximum absorptivity of 99.9% and shows an excellent absorption stability performance with respect to different polarizations and incidence angles. By analyzing equivalent circuit and simulating the the electric field distribution, current distribution on patch and metallic ground plane at the resonance frequency, its physical mechanism of absorption of the electromagnetic wave is theoretically investigated. Experimental results show that the application of the absorber to waveguide slot antenna has hardly influence on antennas radiation performance, while the in-band RCS reduction of antenna is above 3 dB from21 to 21, and the most reduction value exceeds 17 dB at the boresight direction.