The performance of a LiNbO_3 optical modulator with a superconducting electrodes has been studied theoretically as well as experimentally. In the case of velocity matching between optical and signal waves using sheilding plane on top of a coplanar waveguide, numerical calculations of the attenuation constants of both superconducting and normal-conducting transmission lines indicate that the performance of the optical modulator is far superior to that using normal metals with respects of the figure of merit of bandwidth/driving-voltage. Microwave characteristics of a traveling-wave-type LiNbO3 optical modulator with superconducting coplanar waveguide electrode and a shielding plane has been studied experimentally in the low temperature. In the frequency range between dc and 26.5GHz, it is shown that the obtained modulation depth is in good agreement with the theoretically expected one.