The principal refractive indices and the thermal refractive index coefficients for Cr 3+∶Be 3Al 2Si 6O 18 crystal dopted with 1 01 wt.-% Cr 2O 3 have been accurately measured by the auto-collimation method at the wavelengths of 0 488 μm, 0 53975 μm, 1 0795 μm and 1 3414 μm, and at the temperature of 308 2 K, 328 6 K, 359 1 K and 395 4 K, respectively. The sellmeier′s equations and the thermal refractive index coefficients were obtained, and the result was certified to be accurate by error analysis.