Steam generators(SG) of the Korean nuclear power plants were replaced with a new one after a 20 year operation. A metallographic examination of the defected tubes is needed for setting up a guide line for a safe operation of the other SG. A destructive analysis of 50 tubes was addressed, and a relationship between the SCC defect location and defect depth was obtained. Tubes extracted from the retired SG were transferred to a hot laboratory. Detailed nondestructive analysis examinations were taken, then the tubes were destructively examined. Types and sizes of the defects were characterized. A location and depth of the SCC were evaluated in terms of the location and height of the sludge. Most axial cracks were in the sludge pile, whereas the circumferential ones were around the top of the tube sheet(TTS) or below the TTS. Cracks were found with various lengths in a specific zone. Average defect depth of the axial cracks was deeper than that of the circumferential ones.