The extended-defects generated during oxidation in both of nitrogen-doped Czochralski (NCZ) silicon and common Czochralski (CZ) silicon have been investigated by Transmission Electron Microscopy(TEM). It is found that the size of the oxidation-induced stacking faults (OSFs) decreases with the increase of oxidation time in NCZ silicon, and punched-out dislocations could also be observed. While in CZ silicon, there are many polyhedral oxygen precipitates generated, and the size of OSFs increases with increasing oxidation time.