Magnetic multilayers [Ni80Fe20/Cu](15) grown by dc-magnetron sputtering and annealed at 150 degreesC and 250 degreesC were investigated by low-angle and high-angle X-ray diffraction. It was found that the superlattice period decreases upon annealing, while both the interfacial roughness and (111) preferred orientation increase with increasing annealing time. The computer simulation of high-angle X-ray diffraction firstly revealed a very thick intermixing layer located between the Ni80Fe20 and Cu sublayers, and its thickness increases with increasing annealing time. The above results on microstructures of [Ni80Fe20/Cu](15) multilayers were discussed.