Up to now an in-line method for parameter determination of deposited thin polysilicon films is not available. In this paper a method for monitoring the polysilicon deposition process in device manufacturing by variable angle spectroscopic ellipsometry (VASE) is demonstrated. Therefore several polysilicon films are deposited on thermally oxidized  silicon wafers. These samples are characterized by VASE in the optical range of 450 - 850 nm. Parameters are determined by simulation using a multilayer model consisting of air, interface layer (surface roughness), polysilicon, SiO2 and silicon substrate. Different optical models representing the properties of polysilicon are tested. The free parameters are the oxide thickness, the composition and the thickness of the interface layer (air, polysilicon) as well as the thickness and the complex refractive index of the polysilicon layer. Results of the spectroscopic analysis are verified by AFM and SEM measurements. It can be shown that parameters of the deposited polysilicon films, which often could only be determined by complex and destructive off-line analysis methods are also accessible by non-destructive in-line VASE measurements.