Steam generator tubings of a nuclear power plant are very thin to have efficient heat exchange. Steam generator tubings have experienced various types of corrosion damages under the sludge pile and in the tube-tube sheet crevice during operation at high temperature and high pressure. Types of degradation of steam generator tubings found in domestic nuclear power plants are pitting, secondary side stress corrosion cracking caused by Pb, primary water SCC, and fretting in the area in contact with antivibration bar. The ACPD (Alternate Current Potential Drop), which can detect the initiation and propagation of the microcracks, was assembled. The C-ring specimens were machined in order to test a stress corrosion cracking in Pb containing aqueous solution. The denting (stress increasing factor) test on the carbon steel was conducted, and pitting resistance of the different alloy was evaluated. As a result of the chemical cleaning process, it is concluded that the high temperature (125 deg C) chemical cleaning process could be applicable. And it was investigated that there were many cases of cracking of high strength materials used in the reactor internals throughout world. There was a failure in Korea also. (Author).