We have proposed a novel direction-of-arrival (DOA) estimation antenna based on the mono-pulse mechanism. The proposed DOA estimation antenna consists of microstrip antennas and Magic-Ts which generate Σ and Δ signals from received RF waves. As the Both-sided MIC technology is used for the Magic-T and feed circuits, the proposed antenna has a good performance with a simple structure. In this paper, the estimation errors of the proposed antenna are examined when an undesired wave is simultaneously received with the desired wave.