Abstract: In this paper the yielding spread mechanism and the variations of fracture parameters in loading are investigated by bursting experiments of cracked pressure vessels. The different defects in vessels are assessed by the General Yielding Criterion, COD design curve and J-jntegral criterion respectively. It is pointed out that as in the case of wide plate, the COD deep-increasing and freezing phases occur in the vessels also, and that the General Yielding Criterion is an effective means of assessing the defects in pressure vessels while the COD design curve and J-integral Criterion have some limitations in their application.