Based on the theory of four-port resonant filters, the characteristics of filters with ring resonators were numerically simulated by using two-dimensional FDTD method. The relationship between the coupling length of MMI and the filtering features was also simulated . Two narrow-band filters with ring resonators based on GaAlAs/GaAs planar waveguide were designed and fabricated. The measurement on filtering feature was carried out. It coincides well with that from the numeric computation.