We present the theoretical concept of an optical isolator based on a resonance splitting in a silicon ring resonators covered by a magneto-optical polymer cladding. A polymer magneto optical cladding causing a 0.01 amplitude of the off-diagonal element of the dielectric tensor is assumed. Using a perturbation method it is shown that the resonance splitting of the clockwise and counter-clockwise modes increases for smaller ring radii. For the ring with a radius of approximately 1.5μm a 29GHz splitting is demonstrated. An optical isolator is proposed based on a critically coupled ring resonator. © (2011) Trans Tech Publications.