The impact of hot electrons on gate oxide degradation is studied by investigating devices under constant voltage stress and substrate hot electron injection in thin silicon dioxide (2.5–1.5 nm). The build-up defects measured using low voltage stress induced leakage current is reported. Based on these results, we propose to extract the critical parameter of the degradation under simultaneous tunneling and substrate hot-electron stress. During a constant voltage stress the oxide field, the injected charge and the energy of carriers are imposed by VG and cannot be studied independently. Substrate hot electron injection allows controlling the current density independent of the substrate bias and oxide voltage. The results provide an understanding for describing the reliability and the parameters dependence under combined substrate hot electron injection and constant voltage stress tunneling.