Mushroom electromagnetic bandgap structure shows surface-wave bandgap and in-phase reflection, these two characteristics were simultaneously used for designing the waveguide slot array antenna. The surface-wave bandgap was used to suppress the surface-wave in antennas and arrays, which results in radiation performance improvement. In addition, the in-phase reflection could be used to reduce the RCS level of antennas and arrays, so as to improve their total performances. A sample of four-element EBG waveguide slot array antenna was made, and the indexes of mutual coupling, radiation pattern and RCS of the sample were tested. The result is similar with the theoretical expectation, which has effectively reduced the mutual coupling and the RCS level of antennas and arrays.