This paper presents an experimental and numerical study of the mechanical behaviour of Alloy 800 tubes with a circumferential crack-like throughwall flaw. The crack-like throughwall flaw was simulated as a slot fabricated by Electron Discharge Machining (EDM). A digital image correlation (DIC) method is employed to measure the distribution of strain and the deformed EDM slot profile during the tension test. A Finite Element Analysis (FEA) model in ANSYS is then developed to predict the mechanical behaviour of Alloy 800 steam generator (SG) tubing in the tension tests. The FEA model with a verified material stress-strain curve and the calibrated failure criterion is applied to simulate the burst test of SG tubing with a circumferential crack-like throughwall flaw. The maximum allowable size for a circumferential throughwall crack is recommended based on FEA simulations.