The capture cross sections and density‐energy distribution of the trap states at the interface between GaAs epitaxial layers and oxide films grown by anodization in oxygen plasma have been determined by deep‐level transient spectroscopy (DLTS). The capture cross sections are of the order of 10−12–10−13cm2. The state density in the energy space range from 1×1013to 3×1013cm−2 eV−1. It has a peak 0.43 eV below the conduction band edge, but increases again near the valence band edge. The existence of traps in the oxide is also suggested. The activation energies of the emission rates determined by constant‐capacitance DLTS with a small pulse voltage are in good agreement with the surface potentials, and no particular band structure at the interface, such as an interface band, has been found.