A novel heterodyne interferometric ellipsometer was investigated to measure the thicknesses of thin films in the nanometer-accuracy. The depolarization effect of a Nonpolarizing Beam Splitter (NPBS) was characterized by the transmission-induced-retardance (TIR) between p and s components of a polarizing beam, reflectance ratio, transmittance ratio and reflection-induced-retardance (RIR),then a corresponding error model was established. The influence of the depolarization effect and misorientation of a multi-layer dielectric NPBS on the errors of ellipsometric parameters was investigated. Experimental results indicate that the fluctuation of the depolarization effect caused by the environmental temperatures, incident angles and the change of polarizating state has a major impact on the measurement accuracy, and it can not be eliminated by calibration. In order to implement the nanometer-accuracy, the misorientation angle must be smaller than 0.1°. Furthermore, it shows that the NPBS orientation angle errors for recombination of the measurement light and the reference light have more important contribution than another ones to the measurement accuracy. The thickness measurement error of thin film induced by NPBS is about 1.8-2.5 nm，which means that the NPBS is an assignable error source in the Mach-Zehnder interferometric ellipsometer.