The nuclear power plant has lots of pipes that the fluid of high temperature and high pressure flows. Among the pipe materials used at secondary circuit of the power plant the carbon steels are sensitive to corrosion due to their material properties. In this study, both ultrasonic test and acoustic emission test were used to study the corrosion effect for the carbon steel pipe nondestructively. The carbon steel specimens were in the pipe under 473K temperatures and 10MPa pressure conditions for corrosion processing. According to the degree of corrosion the strength of the specimen was evaluated, and the thickness of the corrosion specimens was also measured by using the ultrasonic wave. The experimental results showed that the attenuation factor was also increased as a depth of corrosion increased. The measured depth of the real corrosion by ultrasonic test shows the good agreement with that by an optical microscope. In order to understand the corrosion effect for the failure mechanism of carbon steel, a failure test on the specimen with various corrosion conditions was performed. An acoustic emission technique was also used to evaluate the degree of damage of corrosion specimen in real time. Acoustic emission technique is proved a useful method for on-line monitoring the microscopic failure mechanism and the damage location for the structures.