Relaxor ferroelectric single crystals were intensively investigated due to large piezoelectric effects near the morphotropic phase boundary (MPB). In general, materials with large ferroelastic responses may also have high acousto-optic effects. Especially, both large lattice parameter changes and stress induced phase transition can bring large refractive index changes, which enhances the overall acousto-optic effects. The acousto-optic coefficients π were characterized for 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 (0.65PMN-0.35PT) single crystal under uniaxial stress using Mach-Zender interferometer method. The results show that the crystal exhibits very high acousto-optic values, for example, π33 can reach 15.2×10-12 m2/N, indicating that relaxor ferroelectric single crystals are promising candidates for laser deflectors or acousto-optic modulators.