A novel and simple approach for diplexer synthesis using the composite right/left-handed (CRLH) phase-advance/delay lines combined with a coupler is proposed. By engineering CRLH-based transmission lines with desired phase responses at two arbitrary frequencies of interest, the connected CRLH 90° or 180° coupler is excited in a manner such that signals at designated frequencies are separated to the corresponding output ports of coupler. Based on the present configuration, design complexities such as optimization of the interconnection junctions and the harmonic spurious suppression involved in conventional filter-based diplexers can be avoided. In addition, channel isolation is beneficially achieved from the isolation property of directional couplers. The measured insertion loss and isolation are less than 1 dB and better than 20 dB in dual bands respectively. Good agreement was observed between simulation and measurement.