Abstract: Quasi-Phase-Matched (QPM) linear electro-optic effect is investigated when the incident light is of different polarization. The study shows the electro-optic coupling is influenced by the polarization remarkably. For the case that the QPM condition is satisfied, when the input light is linearly polarized, the energies of o-ray and e-ray can be coupled to each other totally, and the polarization direction can be rotated at any angle by external electric field; when the input light is of ellipsoidal polarization, the coupling between o-ray and e-ray is not complete while the azimuth angle of ellipsoid can also be rotated at any angle; when the input light is circularly polarized, there is no electro-optical coupling between o-ray and e-ray, and polarization of the output light keeps unchanged. For the case that QPM condition is not satisfied, the light propagating along PPLN is just as propagating along a normal birefringent crystal. The electro-optic effect makes no contribution to the birefringence when the poling duty cycle D is equal to 0.5.