The anastomosis strength of vascular substitute and blood vessel is supported not only by the suture thread but also by tissue adhesion due to encapsulation of vascular substitute. In this paper, the in vitro and combined in vivo and in vitro mechanical tests were carried out to estimate the anastomosis strength between a vascular substitute and a blood vessel; good encapsulation of inner wall of the vascular substitute around the anastomosed part was found to be effective to enhance anastomosis strength. Furthermore, these results were compared with the experimental result of the anastomosis being performed by using suture thread only. Concerning the problem of the anastomosed part of the blood vessel with the vascular substitute, both effects mentioned above were found to be important requisites for the design of good functional vascular substitutes suitable for anastomosis. Finally, the significance of the in vitro and combined in vivo and in vitro mechanical tests are discussed.