A novel compact microstrip diplexer for 4G wireless communication system is proposed. The diplexer is composed of two stub-loaded dual-mode resonators (SLDMRs). The main advantage of stub-loaded dual-mode resonator is that a compact narrowband bandpass filter is easily realized and a transmission zero can be realized in the stopband of each channel, which improves the isolation between two channels. Two SLDMRs are then connected to a common port by the feed lines with two long fingers. Coupling scheme of the diplexer is given and analyzed. The diplexer is in final designed at 2.35 and 2.59 GHz for 4G wireless communication. The circuit is compact and occupies about 0.33λg ×0.25λg .