A novel four-port microstrip diplexer for RF interference rejection is presented in this paper by combining two diplexers together. This technique offers the signal isolation of 62.9 dB between transmitter and receiver module, which is the best figure ever reported. The four-port network exploits microstrip structure for the cost reduction, while still offering superior figure-of-merit compared to the existing state-of-the-art diplexers. Finally, four-port microstrip diplexer for RF interference rejection can be used in IMT-2000 applications whereas device miniaturization and low infrastructure cost are required.