In this paper, electro-optic properties of 0.67Pb(Mg1/3Nb2/3)03-0.33PbTiO3 (PMN-33%PT) single crystals under proper AC bias are reported. It is found that PMN-33%PT has an extremely large linear electro-optic coefficient, r33 ~9800 pm/V under proper AC bias. Besides this huge electro-optic coefficient, PMN-33%PT crystals also have very good optical quality and the random scatterings caused by the multiple domains can be totally removed by the AC bias. Furthermore, unlike KTN, PMN-PT has a much higher phase transition temperature (~175°C), which is sufficiently away from the room temperature so that a good thermal stability can be achieved. The combinations of giant electro-optic coefficient, good optical quality, and high thermal stability may make PMN-33%PT the best electro-optic material among all electro-optic crystals developed so far, which could revolutionize applications of electro-optic crystals to telecommunications, medical imaging, et al. This research may also help to understand the mechanism underlying the ultrahigh performance of this new generation of single crystal materials (e.g., PZN-PT, PMN-PT) via the interaction between crystal internal structures and external AC electric field, which may stimulate further interest in the fundamental theory of ferroelectrices under changing electric field that could direct improving the quality of existing ferroelectric crystals and developing new ferroelectric crystals.