A advanced polarization splitter with directional coupler has been proposed for use in integrated optical circuits on LiNbO 3, and a detailed theoretical analysis has been given. The power splitting ratio of the polarization splitter has been calculated by using 2D-beam propagation method(BPM). In theory, the power splitting ratioes of the TM mode and TE mode through the splitter are both above 40 dB. At the same time, some polarization splitters have been actually fabricated. And the power splitting ratioes of the TM mode and TE mode are 28 dB and 25 dB, respectively.