Longitudinal Electro-optic modulation has been demonstrated using poled electro-optic polymer films placed in a Fabry-Perot(F-P) cavity, in which the directions of the poling electric field, the applied electric field and the light traveling are parallel with each other, and perpendicular to the surface of the polymer film. The F-P cavity used here had a fineness of about 18. The thickness of polymer films is about 1μm. The modulation depth about 0.012%, was achieved, in a low ac field of 1V/μm. The results demonstrate that it is possible to achieve electro-optic modulation of poled polymer films and to study the films using F-P cavity.