Formulas for error factors of optical thin film parameters with respect to the ellipsometric angles ψ and Δ, the reflectance R, the angle of incidence, and the thickness of absorbing thin film are presented using the technique of multivarlate error analysis. Accuracies of the measurements of optical parameters can be predicted with the error factors, the azimuth-angle errors, and the reflectance error. In order to determine the optimal measurement configurations, the plots of error factors versus angle of incidence，film refractiye index, and film thickness are used. It is also pointed out that the measurement accuracy of a oortain parameter can not be deduoed and determined by using its sensitivity factors with, respect to the measurable quantities, if two or more unknown parameters have to be simultaneously deduced from the measurements.