Dual mode horn antenna is widely used in reflector antenna systems to illuminate symmetric E/H-plane radiation patterns to a reflector surface. Symmetric E/H-plane radiation pattern can improve the total gain of the reflector antenna system and allows a design of a reflector to be symmetric . Dual mode horn antenna was developed by Potter  and many designs have been proposed to have symmetric E/H-plane radiation pattern -. The mode at the aperture of the dual mode horn antenna is hybrid mode (HE11 mode) which consists of approximately 85% of TE11 mode and 15% of TM 11 mode  and the antenna radiates gaussian-like beam pattern, and therefore the E/H-plane radiation patterns are symmetric ideally. However, if the aperture size of a dual mode horn antenna is increased to attain the desired gain or beamwidth for the reflector antenna system, ideal HE11 mode is difficult to be attained at the aperture because undesired high order modes which lead asymmetric E/H-plane radiation patterns are generated. In this paper, modified dual mode horn antenna with rectangular aperture is proposed to improve the E/H-plane radiation pattern symmetry in oversized aperture horn. Simulation and experimental results show that the proposed dual mode horn antenna has the improved E/H-plane radiation pattern symmetry compared to a conventional circular dual mode horn antenna.