By exploiting the powerful local ability of scanning tunneling microscopy (STM), we studied the ultrathin SiO2degradation and soft-breakdown (SBD) by both DC and dynamic electrical stressing (DES). The results show that the SBD is a local event and characterized by bright spot generation which represents high conductive pathways formed in the oxide. The degradation is not a reversible process and has no observable relaxation effects. By comparing the SBD generation under DC and DES stress, it is found that the SBD density versus stress time can be described by the Weibull statistics. Both the SBD generation rate and final SBD density are lower for DES stressing than for DC stressing, suggesting that a critical energy exists for SBD to be generated.