In order to improve the transmission performance of the conventional Mach-Zehnder interferometer, a novel interleaver based on nonsymmetrical Mach-Zehnder interferometer with adding optical fiber ring resonator to interference arm was presented. The output expression was established and described by using the principle of fiber transmission and the matrix transfer function. Through numerical simulation, the influence of the coupling ratios of the optical fiber ring resonator and the length difference of interference arms on the different bandwidth interleaver was analyzed. The analytical results indicate when the length of interference arms is equal, the port output curves are quite ideal by taking some certain values for the coupling ratios of the optical fiber ring resonator. Between 3 dB points, odd channels have more than 15 GHz passband for 10 Gb/s and the other even channels have more than 60 GHz passband for 40 Gb/s. This kind of interleaver has more advantages as multiplexer/demultiplexer during upgrade from 10 Gb/s to 10 Gb/s plus 40 Gb/s.