Refractive indices and optical transmittance spectra of (1-x)Pb(Zn_1/3Nb_2/3)O₃-xPbTiO₃ (PZN-xPT) single crystals poled along [001] direction have been investigated. Cauchy dispersion equations of the refractive indices were obtained by least square fitting, which can be used to calculate the refractive indices in the low absorption wavelength range. After poled along [001] direction, the transmittance of PZN–12%PT single crystal is more than 65% from 0.5 to 5.8 μm, which is much higher than that of PZN–5%PT and PZN–8%PT single crystals. Optical energy bandgap was obtained from absorption coefficient spectra. Our results show that as the PT content increases, the refractive indices of PZN-xPT single crystals increase, while the optical bandgap decreases.