A compact diplexer based on the slotline-loaded microstrip ring resonator is presented in this letter. The diplexer consists of a microstrip ring resonator loaded with four backside slotlines and a couple of half-wavelength microstrip resonators. On the one hand, two degenerate modes are orthogonally excited in the common ring resonator and they are used to achieve the high isolation between two output ports as demanded. On the other hand, these two excited orthogonal modes work together with their respective half-wavelength resonators to form up the two separate passbands with respect to the two output ports. In addition to its capability in adjustment of passband responses, the proposed diplexer does not need any external matching circuit so as to miniaturize the overall circuit size. The predicted frequency responses are well verified by testing a fabricated diplexer.