In this paper, a third-order planar diplexer is proposed by using common dual-mode rectangular patch resonators. Compared with the traditional diplexer, there is no any connection-oriented junction network, which leads to a small size of the diplexer. There are two orthogonal resonant modes for each resonator. One mode is used as channel I, the other is used as channel II. The coupling coefficient of other channel is analysis. The desired external quality factor can be obtained by selecting the appropriate location of the input port. In addition, a pair of perpendicular slots are at the center of each resonator in order to reduce the insertion loss. For channel I, the measured insertion loss at a center frequency of 2.65 GHz is about 1.6 dB with a fractional bandwidth of 9.3%. For channel II, the measured insertion loss at a center frequency of 2.13 GHz is about 1.9 dB with a fractional bandwidth of 8.6%.