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The off-center displacement of Ni3+ ion in SrO crystal is investigated by analyzing its anisotropic g factors g(//) and g(perpendicular to) by using the perturbation formulas of the g factors for a 3d(7) ion in axial (tetragonal or trigonal) symmetry. In these formulas, the contributions to the g factors from the mixture of different energy states, low symmetry distortion and covalency effect are included and the parameters related to these effects can be obtained from the optical spectral and local structural parameters of the studied system. Based on the studies, it is found that the Ni3+ impurity may take an off-center displacement by about 0.23 Angstrom along the [1 1 1] direction and result in a trigonally distorted [NiO6](9-) cluster. Interestingly, the observed Ni3+ center has the unusual T-4(1) ground orbital triplet due to the weak crystal field of the host SrO, which is different from the usual E-2 ground orbital doublet for Ni3+ under strong octahedral fields. The reasonableness of the results is discussed. (C) 2003 Elsevier B.V. All rights reserved.