The numerical simulation of the stretch flange forming operation of Al-Mg sheet AA5182 was conducted with an explicit finite element code, LS-DYNA. A Gurson-Tvergaard-Needleman (GTN)-based material model was used in the finite element calculation. A strain controlled void nucleation rule was adopted with void nucleating particle fraction measured directly from the as-received Al-Mg sheet. Parametric study was performed to examine the effect of void nucleation strain on the predicted onset of ductile fracture. Critical porosity levels determined through quantitative metallurgical analysis were adopted to predict the commencement of void coalescence in the GTN model. The numerical results were compared to the experimental ones and an applicable void nucleation strain was suggested.