Abstract: In this paper, the effect of flaw on the strength of steam generator (SG) tubes and the plugging criteria were studied. The test device used for measuring plastic limited load and bursting pressure of small diameter tubes was developed and experiments of Inconel 690 SG tubes with local wall thinning and crack flaws were carried out, meanwhile, finite element method was used to estimate plastic limited load and bursting pressure. Based on these methods, a new plugging criteria for SG tubes with two-level assessment was proposed. In this criteria, the thickness, longitudinal length and circumferential length of flaw were synthetically used to assess the safety of SG tubes.