The stress-strain relation of the autogenous vein grafts were investigated 24 weeks postsurgically. Canine femoral veins were excised, reversed, and grafted to femoral arteries in the end-to-end anastomosis fashion. The stress-strain relations were reduced from uniaxial tensile tests of the longitudinal and circumferential specimens of the vessels. Results show that the longitudinal stress-strain relations of vein graft (24 wks postsurgically) and the normal vein can be expressed in form of T=C1(e-1) for small stress (T<10kPa), or T=C2e-β for large stress (T≥10kPa); The circumferential stress strain relations can be expressed in form of T=C3(e-1.Both the longitudinal and the circumferential stress-strain relations of the vein graft is shifted left-ward. Both structural stiffness and material stiffness of the vein graft is larger than the normal vein. e.g. longitudinally, the values a2(mean±SD) are 34.06±7.30 and 19.21±2.35 for the vein graft and normal vein respectively, while circumferentially, the values of a3(mean±SD) are 55.76±20.16 and 27.56±2.82 for the vein graft and the normal vein respectively. In conclusion, the stiffness of the vein graft increases postsurgically, and the changes are mechanically anisotropic-differing in the longitudinal and circumferential directions.