In this work, the formation of nickel germanide is investigated by spectroscopic ellipsometry. The phase transition and thickness of nickel germanide were identified nondestructively by spectroscopic ellipsometry. The results were compared with traditional analytical tools such as four point probe, Rutherford backscattering spectrometry, and X-ray photoelectron spectroscopy, and showed good agreement. The optical properties (refractive index) of nickel germanide were obtained from spectroscopic ellipsometer measurements and applied to the thickness measurement of nickel germanide successfully.