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The defect structure and the anisotropic g factors of a tetragonal Ni3+ center in PbTiO3 are theoretically investigated from improved perturbation formulas of the g factors for a 3d(7) ion with low spin S = 1/2 in tetragonally elongated octahedra, established in this work. Based on the studies, the distance between the impurity Ni3+ and the center of the oxygen octahedron is found to be about 0.14 A, which is smaller than that (approximate to 0.3 angstrom) for the host Ti4+ site due to the inward shift (approximate to 0.16 angstrom) towards the center of the oxygen octahedron. The theoretical g factors based on the above defect structure agree well with the observed values.