An interleaver combining ring resonator with Mach-Zehnder interferometer (MZI) is proposed. Based on the phase modification provided by the ring resonator, a maximum flatness filtering response is obtained by optimizing the coupling angle of resonator. Compared with the traditional MZI, the isolation in stopband (>38 dB) and the rolloff in transition band are strengthened, but also the dispersion in passband is reduced to ±10 ps/nm in the center frequency range of 15 GHz which eliminate the distortion of transmission signals. Furthermore, the influences of ring circumference (Lr) on length difference (ΔL) of MZI arms are investigated, and the bandwidth allocation of odd-even ports is directly determined by Lr/ΔL. The result demonstrates that a novel interleaver with 1∶2 asymmetric bandwidth allocation can be achieved by adjusting Lr/ΔL=1/2. This kind of interleaver will make system upgrade from 10 Gb/s to 10 Gb/s+40 Gb/s in a more simple and flexible way.