The perturbation formulas of the EPR g factors g x , g y and g z for a 4f 11 ion in orthorhombic symmetry are established. In these formulas, the contributions to the g factors arising from the second-order perturbation terms and the admixture of various states are taken into account. The above formulas are applied to the orthorhombic Er 3+ centers in YAG, YGG and LuGG garnets. By analyzing their g factors, we find that the polar angles h 0 j of the impurity-ligand bonds related to the Z axis of the crystals in these Er 3+ centers are smaller than the host angles h j in pure crystals, whereas the azimuthal angles / 0 j related to the X axis in the impurity centers are larger than the corresponding / j in the hosts. The g factors and their anisotropies Dg [= g z Γ (g x + g y )/2] and dg (= g x Γ g y ) for Er 3+ in the garnets based on the above local structural parameters are in good agreement with the observed values. In addition, the angular distortions Dh (ΒΌ h 0 j Γ h j ) or D/ (ΒΌ / 0 j Γ / j ) obtained by analyzing the EPR data in this work are different from Dh 2 or D/ 2 (for only the second set of ligands, i.e., j = 2) based on the superposition model crystal-field analysis in the previous work. The validity of the results is discussed.