The principal refractive indices of yttrium aluminate crystal doping with 1-wt. % neodymium (Nd: YAP) have been measured by a minimum deviation method for the wavelengths 0.5398, 0.6328, aAnd 1.0795 μm in the temperature range of 289-435 K. The thermal refractive index coefficients have been obtained at these wavelengths, and they are all positive and slightly decrease with increased wavelength. The constants of the single term Sellmeier equation are also given for the 273-420 K temperature range. These constants are in linear relationship with temperature in the 4 × 10-3 error range. © 1990 Optical Society of America.