Abstract: The functional capability of piping systems is different from the pressure boundary integrity of piping system, and United States Nuclear Regulatory Commission (NRC) specified two evaluation criteria for the piping functional capability. To explore the origin and the application of functional capability evaluation criteria, and based on the research of classical literatures about the content of functional evaluation criteria, this paper expounds the origin and the basis of two kinds of evaluation criteria, analyzes the characteristics of two kinds of evaluation criteria, and proposed  notes on the application of two kinds of evaluation criteria. At the end of this paper, through an example of piping functional capability assessment, two strategies for the application of functional assessment criteria in different nuclear plant design stages are proposed. For newly built nuclear power plants, C level stress limit shall be used as possible to ensure the functional capability of piping systems. For the nuclear power plants that had been built,  D level stress limit and five additional conditions can be applied to ensure the functional capability of piping systems.