The reactive double bonds were introduced to the backbone of polyurethane (PU) and then N-vinyl pyrrolidone (NVP) which has hydrophilicity and biocompatibility was grafted to improve the blood compatibility. Also, the mechanical properties were varied by adjusting the amount of double bonds introduced. When a few double bonds were introduced to the backbone of PU, mechanical strength was little weakened in spite of the increase of grafting efficiency. Blood compatibility was evaluated by the film depositing method and the number of platelets was measured by Coulter Counter. Appearance of platelets adhered on the polymer surfaces was observed by scanning electron microscopy. Contact angles were measured by means of captive bubble technique and interfacial free energies of water-solid interfaces were calculated to estimate the thromboresistance. It was consequently found that the interfacial free energy approached conspicuously to zero with the increase of degree of swelling.