Abstract: In the pre-service and in-service inspection of nuclear power plant equipment, the defects to be detected in the unclear class weld required for rigorous analysis and evaluation to ensure the safe operation of the nuclear plant. This paper describes the general process of defect display and takes the ASME and RSE-M codes as example to analyze the properties of defects, size normalization, correlation, and acceptance and so on. The difference between the two codes has been also summarized in the paper.