A method for multiple angle of incidence (MAI) ellipsometric analysis in determination of double-layer optical thin film parameters is described. The error factory and the choice of the optimal measurement conditions of optical thin film parpmeters in MAI ellipsometric analysis are dicussed. It is showed that the method enables the determination of all the optical parameters and thickness of double-layer film system with a high accuracy if the systems are of sufficient thickness. This fact is illustrated with tha samples of Si/SiO2/Si3N4 and Si/SiO2/poly-Si systems. The method can be extended to analyzing the mutilayer optical "bhin film systems.