We propose an interleaver based oil the Mach-Zehnder interferometer with a resonator incorporated in one ann. By using the method of signal flow graph, we get the simple closed-form expressions for transmission of this interleaver. The result indicates that the widths of 0.5 dB passband and 25 dB stopband of the interleaver are improved remarkably, which are much wider than that of the conventional Mach-Zehnder interferometer. The interleaver proposed has an ideal rectangular spectral response. Finally, we have analyzed the influence exerted by the parameters on its transimission characteristics and found that the transmission spectrum of this device depends highly oil the intensity coupling coefficient of the coupler and fiber loop length.