The purpose of this paper is to discuss the plastic deformation and pressure reversal of pipe during hydrostatic testing. The higher the test pressure, the smaller the number of defects that remain undetected. In order to study the effect of hydrostatic testing, full scale burst experiments were carried out with API 5L X65 SSAW pipes of 660mm outside diameter and 12.5 mm wall thickness. Nine spiral oriented deflects had been machined in the weld. Deformations of deflects were measured. The pressure reversal was computed. It can be concluded that if a hydrostatic test can be successfully accomplished without the failure of any defect, the likelihood of a pressure reversal will be extremely small.