The hydrolytic degradation in vitro of polydioxanone suture(PDS) was studied by examining the change of tentile strength,weight loss,molecular weight,macromolecular orientation,and the level of crystallinity of the suture samples.It was found that the breaking stress decreased with decrease of molecular weight and degree of macromolecular orientation.The significant decrease of molecular weight while the weight loss changing a little for the PDS suture in the period of four weeks in vitro degradation proved that the reactivity for hydrolysis of ester linkages is the same, whether in the middle of the chain or in the chain end,and only depends upon the accessibility of water molecule.The result of WAXD and DSC showed that the original crystaliline region substantially remained unchanged during the initial four weeks in vitro degradation,and the new ordered regions appeared in the amorphous area.These results could be explained by the change of macromolecular structure of sutures.The whole process of hydrolytic degradation in witro for PDS samples is similiar to that for PGA suture.