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A compact microstrip diplexer designed with a dual-bandpass filter and two simple matching circuits has been proposed in this paper. By using only a single dual-bandpass filter, the proposed diplexer may be made compact and miniature when compared with the conventional configuration of the diplexer based on combination of two separate bandpass filters. In this study, the dual-bandpass filter is realized using parallel coupled microstrip lines and open-loop stepped-impedance resonators (SIRs) loaded with two shunt open stubs, and the matching circuits are developed based on the single-shunt-stub tuning and the double-shunt-stub tuning which is called π type circuit. The diplexer is designed to support dual-band operation at 2.4-2.5 GHz and 4.9-5.8 GHz frequency bands which are the frequencies for WLAN applications. The measured results agree well with the simulated results which show that this microstrip filter has excellent performance on return loss, insertion loss and isolation.