A novel integrated dense wavelength division multiplexing interleaver scheme is presented based on phased-array (PHASAR) wavelength demultiplexer with multimode interference (MMI) couplers. MMI-PHASAR interleaver with simple structure and compact size can realize narrow channel spacing through simple design procedure. And it is convenient for integration with integrated planar waveguide multiplexer/demultiplexers. A 25/50-GHz MMI-PHASAR interleaver is designed and the transmission characteristic is investigated by beam propagation method.