Principal refractive indices and its thermal coefficients of LiNbO3 crystal doped with 5 mol% MgO were carefully measured at the wavelengths of 0.5397 μm, 0.6328 urn, 1.0795 μm and 1.3414 μm and the temperature of 293 K, 348 K, 389 K and 428 K in the Pervious papers[1]. On the basis of measured values and modified Sellmeier's equatins, the expressions of thermal refractive index coefficients of this kind of crystal are derived in this paper. Using these expressions, the thermal refractive index coefficients in the temperature range of 293~428 K and the wavelength range of 0.5398～1.3414 μm have been calculated. It is shown that the maximum relative deviations between the calculated and experimental values is 12%. Based on the calculated value of thermal refractive index coefficient with maximum relative deviation, the noncritical phase matching temperature for 1.0795 μm is calculated to be 382.4 K, only 6 K difference with the experimental value. The expressions are useful for design of the nonlinear optical device using this kind of crystal adopted noncritical phase matching technology.