By analyzing the EPR parameters D and (a−F) of Fe 3+ ion located at octahedral site in yttrium gallium garnet, the local lattice structure around Fe 3+ ion in the crystal has been investigated on the basis of the complete energy matrices for a d 5 configuration ion in a trigonal ligand field. It is shown that there exists a slight expansion distortion in the local lattice structure and the distortion is different at 4.2 and 295K. This result may be attributed to the fact that the radius of Fe 3+ ion is larger than that of Ga 3+ ion, and the Fe 3+ ion will push the two oxygen triangles to move upwards and downwards, respectively, along C 3 -axis when the temperature decreases. From EPR calculation, the local lattice structure parameters R=2.0068Å, θ=51.4691° at 295K and R=2.010Å, θ=51.3706° at 4.2K for octahedral Fe 3+ centre in yttrium gallium garnet are determined.