By introducing magneto-optical garnets with high Faraday rotation and low optical loss in a ring resonator, a nonreciprocal phase shift is generated to split the resonance wavelengths of clockwise and counter-clockwise modes under magnetic field. There are three main applications based on this nonreciprocal effect, optical isolators, optical circulators, and tunable optical filters. The concept of the tunable filters and the design of optical isolators for TE and TM modes are described in the paper. With proper optical ring isolator configurations, optical circulators can be realized.