Inverse photonic nickel-based crystal films formed by electrocrystallization of metal inside the voids of polymer artificial opal have been studied using the microradian X-ray diffraction. Analysis of the diffraction images agrees with an face-centred cubic (FCC) structure with the lattice constant a₀ 650 10 nm and indicates two types of stacking sequences coexisting in the crystal (twins of ABCABC... and ACBACB... ordering motifs), the ratio between them being 4:5 The transverse structural correlation length L_tran is 2.4 0.1 μm, which corresponds to a sample thickness of 6 layers. The in-plane structural correlation length L _long is 3.4 0.2 μm, and the structure mosaic is of order of 10°.