In this paper, strain relaxed InGaAs/GaAs strained-layer superlattice has been studied by X-ray double-crystal diffraction and topography. By simulating the double-crystal rocking curves using X-ray dynamical scattering theory, important imformation such as the structure, the mechanism and degree of strain relaxation, the misorientation between the superlattice layers and the substrate and misfit dislocation density, and so forth are obtained. The misfit dislocation distribution at the interface between the superlattice layers and the substrate and in the superlattice layers are observed by double-crystal topography.