The optical spectrum and the local defect structure of tetrahedral (FeO 4 ) 5− clusters are investigated in yttrium gallium garnet (YGG) and lutetium gallium garnet (LGG) crystals by means of the 252×252 complete energy matrices for d 5 configuration ions in tetragonal ligand field. The results show that the local defect structures around tetrahedral Fe 3+ centers display an expansion effect. Simultaneously, the (FeO 4 ) 5− clusters in the two different crystals have very similar local structures, which are close to those in YIG garnet crystal. Finally the relative curves of the zero-field splitting energies ΔE 1 and ΔE 2 in the ground state 6 A 1 varying with the structure parameter θ are plotted.