The chemical modulation, structure and strain in Ni/Ti multilayer thin films are analyzed using X-ray diffraction theories. The repeat period of the multilayers used in this study ranges from 1.3 x 10(-9) to 12.8 x 10(-9) m. The composition modulation is obtained by using a kinematical theory of X-ray diffraction. Interplanar spacings and the strain within each atomic layer are found by iteratively fitting the experimental X-ray diffraction curves with the simulated one from a dynamical theory of X-ray diffraction.