A novel characterization method is proposed to extract the optical frequency field-effect mobility (μop,FE) of transparent conductive oxide (TCO) materials by a tunable silicon microring resonator with a heterogeneously integrated titanium-doped indium oxide (ITiO)/SiO2/silicon metal–oxide–semiconductor (MOS) capacitor. By operating the microring in the accumulation mode, the quality factor and resonance wavelength shift are measured and subsequently used to derive the μop,FE in the ultra-thin accumulation layer. Experimental results demonstrate that the μop,FE of ITiO increases from 25.3 to 38.4cm2?V-1?s-1 with increasing gate voltages, which shows a similar trend as that at the electric frequency.