The refractive indices and their dispersions in 0.62Pb(Mg1/3Nb2/ 3)O 3-0.38PbTiO3(PMNT62/38)single crystals were accurately mea sured with the min imum deviation method at 20℃.The parameters of modified Sellmeier's equation fo r both no and ne are obtained.We relate the refractive ind ices to energy-ban d structure of oxygen-octahedron ferroelectrics and the Sellmeier optical coeffi cients,E0=5.50eV,λ0=0.226μm，Ｓ０=1.004×1014m-2 and Ed=28 .10eV for n0 and Ｅ0=5.57eV,λ0=0.223μm，S0=1.017×1014m-2 and Ed=28.10eV for ne were obtained through fitting the si ngle-term oscillat or equation.For materials with ABO3 perovskite structure,the octahedr al BO6 unit governs the lower lying conduction bands and the upper valence bands and ma kes a substantial contribution to the refractive indices.