In this paper we report the principal refractive indices n0, n , and their thermal coefficients of LiNbO3 crystal doped with 5 mol % MgO. The principal refractive indices of the crystal have been measured by selfeolliniating method for the wavelengths of 0.5398 μm,0.6323 μm, 1.0795μm and 1.3414 μm in the temperature range of 20°-160℃. The thermal coefficients of the refractive index have been obtained at these wavelengths and constants of the modified Sellmeier's equations are also evaluated in the above mentioned temperature range. The results are used to the values obtained are in good agreement with the experimental results, similarly calculation of noncritical phase matching temperature for SHG gives a result which also agrees with the experimental result fairly well.