Quadratic electro-optic effects in the system Pb(Zn1/3Nb2/3)O3–PbTiO3 (up to 40 mole percent PbTiO3) are measured by applying dc electric field in the temperature region above Curie temperature. The crystals are prepared by a flux method with PbO. For Pb(Zn1/3Nb2/3)O3, the values of g coefficients are g11-g12=0.011 m4/C2 and g44=0.007 m4/C2 at λ=632.8 mµ. With the increase of PbTiO3 mole percent, the g values increase, approaching 0.06 m4/C2 at PbTiO3. Some discussions are presented on the optical properties of the crystals.