Op tical heterodyne interferometry together with reflective ellip sometry, a fast measurement technology with high anti-interference performance was app lied to nanometer film. The nonlinear error of frequency mixing in this measurement technology was analyzed and calculated, and it was pointed out that the ellip tic polarization producing from Zeeman laser and the error ofwave plate was the key issue. The definition of assessment factorwas given for error estimation, which are of great benefit to the design of interferometric ellip sometry system.