Multiple-angle incident (MAT) ellipsometry is used to study the optical properties of both amorphous silicon and poly-Si after different heat-treatments. At first, the error sensitivity of MAT ellipsometry has been studied and the optimum conditions to achieve the best precision are obtained. The optical constants and thickness of amorphous silicon deposited by LPCVD at 550°C are changed significantly after sample annealing. The optical constants of the amorphous silicon reached the values of single-crystalline silicon after annealing for both the high and low temperature treatments. A thickness shrinking phenomenon is observed when amorphous silicon is transformed to the polycrystalline form. This phenomenon is confirmed by cross-sectional TEM photography. The thickness shrinking ratio is 3.5 to 6.0%.