The wave absorbers composed of a periodic structure of the electromagnetic bandgap structures have been investigated in order to suppress the interference. Thin metasurface wave absorbers with stable operation for absorbing frequency at oblique incident angle have been required. Typical unit cell configurations for the metasurfaces are mushroom structure and spiral structure. In this paper, the monostatic RCS reduction characteristics of the flat and curved metasurfaces with 0.1-mm thickness are investigated. The RCS reduction less than −10 dB can be achieved in the flat and curved unit cell structures at around 10.2 GHz. In addition, the curved wave absorber shows similar RCS reduction characteristics to those of the flat one. The validity of the simulation is verified through the experiment.