Iron native oxide layers with unknown optical constants and film thicknesses on an iron substrate with unknown optical constants were characterized by spectroscopic ellipsometry (SE) using an equivalent-substrate method.Iron films deposited on silicon wafer by magnetron sputtering and exposed to air at room temperature were measured by spectroscopic ellipsometry at different time points from ten minutes to seven months.Pseudo optical constants were extracted from the first measured data and were introduced into the modeling work of subsequent measured data as the optical constants of an equivalent substrate.After obtaining the optical constants of the subsequent native oxide layer,they were employed in the modeling work of the first measured data and the optical constants of the iron substrate and the film thickness of the first native oxide layer was eventually obtained.