We report a dynamic and microscopic investigation of electrical stress induced defects in metal-oxide-semiconductor (MOS) devices with high-k gate dielectric by using electron-beam induced current (EBIC) technique. The correlation between time-dependent dielectric breakdown (TDDB) characteristics and EBIC imaging of breakdown sites are found. A systematic study was performed on pre-existing and electrical stress induced defects. Stress-induced defects are related to the formation of electron trapping defects. The origin of pre-existing defects is also discussed in terms of oxygen vacancy model with comparing different gate electrodes.