An all-fiber optical interleaver with almost rectangular wavelength response is developed using a new configuration. The device consists of a 2×2 directional fiber coupler and a fiber ring resonator; the general formulas are derived with the transfer matrix method, and explicit expressions for the power are given. The characteristic parameters, such as amplitude flatness, bandwidth, and phase group delay, are analyzed and optimized. The simulation indicates that the bandwidth of 0.5-dB passband and 25-dB stopband of proposed interleaver are simultaneously remarkably improved, which is much wider than those of the conventional Mach-Zehnder interferometer (MZI) and two-stage MZI (TS-MZI). Further analysis indicates that the influence of the propagation loss on extinction ratio can be effectively reduced compared with the interleaver based on an asymmetrical MZI with a ring resonator in one arm.