Abstract: The microstructure of post-irradiated EG-F2 N steel for reactor pressure vessel weld material after impact was observed by scanning electron microscopy（SEM） and metallographic microscope（OM）. The results show that the reason of the abnormal impact data was related to the plenty of holes on the fracture surface of samples. With the increasing of the number of holes, the effective work area was decreased and the stress concentration was appeared, thus decreasing the strength and toughness of welded specimen significantly. Additionally, a few Al₂O₃ impurities which level is considered less than 0.5 were found in some small holes. So, the main cause of low impact energy of welded specimen is not the Al₂O₃ impurities, but the big hole which is caused by improper operation of welding process.