Y₃(Al,Ga)₅O₁₂:Tb thin films were grown on Si(100) substrates in an Ar working atmosphere by using the pulsed laser deposition (PLD) technique. The Y₃(Al,Ga)₅O₁₂:Tb target was ablation deposited onto a Si(100) substrate using a 266nm Nd:YAG laser. The influence of post deposition annealing temperature (1073K to 1473K) on the excitation and the emission bands, and the crystal structure of the thin film were monitored. X-ray diffraction (XRD) and photoelectron spectroscopy (XPS) depth profiles of the thin films indicate that there were annealing induced changes in the crystal structure and chemical composition causing changes in the excitation bands. These changes (structure and composition) are attributed to interdiffusion of atomic species between the substrate and the Y₃(Al,Ga)₅O₁₂:Tb³⁺ thin film. The XRD and XPS data confirm that after annealing, Y₃(Al,Ga)₅O₁₂:Tb³⁺ was converted to Y₂Si₂O₇:Tb³⁺. A change in the relative ratios of the excitation band intensities was measured. Atomic force microscopy (AFM) showed that topographical changes also occurred during the annealing process. Thermoluminescence (TL) glow curves of the Y₃(Al,Ga)₅O₁₂:Tb³⁺ thin films before and after annealing, indicated the presence of different types of traps resulting from the change on the structure of the thin films.