The Y3Al5−xGaxO12(YAGG):Ce3+-Cr3+ persistent phosphor is one of the materials in which Cr3+ ions act as electron traps. The possibility of electron traps by other transition metal ions (TM3+, TM = Sc, Ti, V, Cr, Fe) was investigated and those electron trap depth was compared with each other. In the thermoluminescence (TL) glow curves, the YAGG:Ce3+ samples co-doped with different TM3+ ions show different TL glow peak temperatures (i.e. different electron trap depth). The estimated vacuum referred binding energy of TM2+ from the trap depth shows a zig-zag curve, which is found to be originated from the different 3d electron energies affected by a number of d electron, nuclear charge and crystal field splitting. Utilizing the obtained TM2+ zig-zag curve, a new persistent phosphor of Y3Al2Ga3O12:Ce3+-Sc3+ and a photostimulated phosphor of Y3Al2Ga3O12:Ce3+-V3+ were successfully developed.