Abstract: A method of measuring the thickness of an epitaxial layer is disclosed. The method is particularly useful in measuring the epitaxial layer with a doping concentration lower than or similar to the substrate on which the epitaxial layer is formed. The method uses a non-single crystal layer previously formed on the substrate before forming the epitaxial layer over the substrate so that the portion of the epitaxial layer on the non-single crystal layer will be polycrystal. To obtain the thickness of the epitaxial layer, thicknesses of the polycrystal layer and the non-single crystal layer as well as the thickness difference between the polycrystal layer plus the non-single crystal layer and the epitaxial layer are measured. The thickness of the epitaxial layer equals the result of the total thickness of the polycrystal layer plus the non-single crystal layer minus the thickness difference between the polycrystal layer plus the non-single crystal layer and the epitaxial layer.