A new phase‐tuning method for an optical directional coupler fabricated in ac‐plate LiNbO3by TiO2diffusion is presented. The method consists of applying a bias voltage to an additional electrode set closely outside the modulation electrodes. The propagation constants of the two waveguides could be made equal by a bias voltage of 5 V. The modulation voltage of 8 V was unaffected by the bias voltage.