The electro-optic effect is an important mechanism for actively tuning the refractive index of materials. This effect has various important applications in communication, switching, modulation, and nonlinear optics. This research measured the quadratic electro-optic coefficient for a graphene oxide (GO) film with ellipsometry spectroscopy. The results show that this coefficient is about three orders of magnitude greater than that of other materials. The GO film with its giant electro-optic Kerr coefficient can improve devices based on this effect. For example, it can decrease power consumption and the complexity of these devices due to the need for a lower electric field. In addition, birefringence is obtained of about Δn = 0.08 at 730 nm, which can lead to promising improvements in commercial devices, such as the reduction of working voltage below 10 V.