The analytical expressions for thickness and delta (delta = 1 - refractive index) of an interface layer from x-ray reflectivity measurements are obtained for an epitaxial layer/interface layer/Si substrate system. The contaminated interface layer between the defective epitaxial layer and Si substrate is detected by the x-ray reflectivity. The stacking fault density (SFD) in the epitaxial layer is correlated with the delta of the interface layer. There is a linear relation between the delta and oxygen concentration of the interface layers. The delta of the interface layer is less than that of Si which is mainly due to oxygen in the interface layer. The thicknesses of the interface layers are about 1.4 nm and they do not correlate with the SFDs in the epitaxial layer. The thickness of the interface layer obtained by x-ray reflectivity agrees with that from TEM observations.