Abstract: A high isolated 4-port diplexer based on microstrip line is presented in this paper. The diplexer is composed of two pairs of filters operating different frequency, each pair include 2 filters with phase of 90° and -90°. The diplexer can achieve an isolation enhancement compared with some reported 3-port diplexers, resulting from the 4-port circuit topology and difference of phase 180° suppresses the leakage between the two channels. The filter of proposed 4-port diplexer is based on half-wavelength open-loop resonator structure, which enables the inverter without incurring any extra circuit footprint. For validation, the diplexer with operating frequencies of 1.8 GHz and 2.1 GHz is designed, fabricated and measured. The experimental results show that the 4-port diplexer has good in-band performance and high isolation of 50.9 dB and 43.5 dB. Compared to the traditional 3-port diplexer, the 4-port diplexer increased by 9.7 dB and 12.8 dB respectively, which indicates that the novel diplexer increased the isolation significantly.