Inverse photonic nickel-based crystal films formed by lectrocrystallization 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 Ltran is 2.4 ± 0.1 mkm, which corresponds to a sample thickness of 6 layers. The in-plane structural correlation length Llong is 3.4 ± 0.2 mkm, and the structure mosaic is of order of 10