A wavelength splitter with ultra-compact and simple structure is proposed and analyzed by using both plane wave expansion (PWE) method and finite difference time domain (FDTD) method. The device is based on directional coupling between two parallel lithium niobate (LiNbO3, LN) nanowire optical waveguides. The wavelength splitter with a coupling region length of 5 μm can separate 1.31μm and 1.55 μm wavelengths for corresponding outputs with transmittance higher than 97%.