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The local structure and the electron paramagnetic resonance (EPR) g factors for the tetragonal Er3+ center in KMgF3 are theoretically studied by using the perturbation formulas of the g factors for a 4f(11) ion in tetragonal symmetry. In these formulas, the contributions to the g factors from the second-order perturbation terms and the admixtures of various states are taken into account. Based on the studies, the impurity Er3+ is expected to occupy the octahedral Mg2+ site, associated with an oxygen ion substituting for one of the nearest F- (i.e., O-F) in the C-4 axis, due to charge compensation. Because of the electrostatic attraction of the compensator O-F, the Er3+ ion is found to take an axial displacement Delta Z (approximate to 0.07 angstrom) towards the compensator along the C-4 axis. The calculated g factors based on the above displacement Delta Z show reasonable agreement with the observed values. (C) 2005 Elsevier B.V. All rights reserved.