By analyzing the EPR zero-field splitting parameters of Fe3+ located at tetrahedral site in Tl2MgF4 and Tl2ZnF4 crystals, the local lattice structure for Fe3+ doped in Tl2MgF4 and Tl2ZnF4 crystals is theoretically studied by diagonalizing the complete energy matrices for a d(5) configuration ion in a tetragonal ligand-field. From the calculation, the local lattice structure parameters R-perpendicular to = 1.8914 angstrom, R-parallel to = 1.84699 angstrom and R-perpendicular to = 1.88229 angstrom, R-parallel to = 1.84490 angstrom for tetragonal Fe3+ center in Tl2MgF4 and Tl2ZnF4 crystals are determined, respectively. This result suggests that the ligand-fluorines on the C-4 axis deviate easily in both two crystals, which is consistent with the conclusion drawn by earlier workers. (c) 2007 Elsevier B.V. All rights reserved.