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We investigate the electrical properties of bonded wafers using deep-level transient spectroscopy (DLTS). A directly bonded silicon wafer and bonded silicon on insulator (SOI) wafers with different interfacial oxide thicknesses are evaluated. In a directly bonded wafer, one unstable trap and two traps (Ec-0.16 eV, Ec-0.24 eV) are detected. They exist within about 20 mu m of the bonded interface in both the active layers and the substrates. In the bonded SOI wafers, traps with the same activation energies as those in the directly bonded wafer are observed, and their densities depend on the thickness of the bonded interfacial oxide. The density of the trap with the energy level of Ec-0.16 eV increases with the interfacial oxide thickness, and the density of the trap with Ec-0.24 eV decreases.