The measuring condition for principle angle in spectroscopic ellipsometry is analyzed. As the incident angle is equal to principle angle θ p, the phase angle of ellipsometry parameters is 90°. It gives a way to obtain θ p from numerical calculation and analytic equation as the dielectric function of the material is known, and the amplitude ρ p0 of ellipsometry parameters can be calculated accordingly. Higher precision of data can be acquired as measuring at incident angle of θ p. The experimental results of Δ p and ρ p0 correspond very well with the calculated ones. The calculation formulas and methods given in this work can be applied to other spectroscopic experiments.