An x-ray-diffraction method for determining the thickness of multiple layer thin films grown on a single-crystal substrate is presented. The equations, based on the kinematical theory of x-ray diffraction and the mosaic crystal model, were developed. As an example of the application of the method, thicknesses of a double heterostructure system, namely AlAs/AlGaAs/GaAs, were determined. Good agreement was obtained between the results from x-ray measurement and scanning electron microscopy data, demonstrating the high precision of this technique.