An ultracompact optical directional coupler based on LiNbO 3 (LN) photonic wires is proposed and analyzed by using finite difference time domain (FDTD). Three simulation results are obtained: (1) Coupling lengths of the directional couplers as a function of the distance S c between two axes of two parallel waveguides at waveguide width of 0.5μm for quasi-TE (qTE) and quasi-TM (qTM) modes, respectively. (2) Coupling lengths of the directional couplers as a function of waveguide width for qTE and qTM modes, respectively. (3) Crosstalk of the directional coupler as a function of wavelength. The proposed coupler can be used for highly integrated optical circuits based on LN photonic wires.