A controlled cross-linking polymerization system was designed, and soluble chemically cross-linked polyurethane was synthesized using laurylamine, n-octylamine, n-pentylamine, and ethylenediamine chain extenders. The mechanical analysis showed that the polyurethane materials synthesized in this paper have very excellent mechanical properties with a breaking elongation of 1914% and a tensile strength of 4303 N/cm2. Such good mechanical properties must enable it to have good longevity when used as biomaterials. The polyurethane materials with n-pentylamine and n-octylamine chain extenders show reduced platelet adhesion than that with an ethylenediamine chain extender after sustaining 200 000 times of load cycles, indicating that polyurethanes introduced with an alkyl side chain onto the hard segments keep good antithrombogenic properties after sustaining load cycles. This might be because the hard segments are shielded by the alkyl side chain when the micro-phase-separation structure is destroyed in the repeated deformation of the polyurethane materials. The present investigation reveals that the influence of introducing long alkyl side chains into the backbone of the polyurethane macromolecule has been shown to reduce platelet deposition and to enhance in vitro albumin adsorption. However, in this paper, it has been observed that the polyurethane material introduced with a proper-length alkyl side chain onto the hard segment has the best antithrombogenic properties after the fatigue test. 1. Introduction Polyurethanes (PUs) have been used for various biomedical applications, viz., cardiovascular devices, in comparison to the other elastomers due to appreciable physical and mechanical properties and biocompatibility.1,2 During the past few decades, PUs have been widely used for biomedical applications such as vascular prostheses, endotracheal tubes, pacemaker lead wire insulation, catheters, and artificial hearts due to their excellent mechanical properties and comparatively good tissue and blood compatibility.1,2 For most cardiovascular products, in which polyurethanes are incorporated as a structural or coating material, it is essential that the material should be not only stable for a prolonged period but also blood-compatible if inserted into the blood stream. It has been reported that PUs show a high affinity for albumin adsorption and low platelet reactivity, and introducing an alkyl side chain or incorporating phospholipids or phosphatidylcholine analogues into the polyurethane backbone has been shown to improve its blood compatibility.3-22 However, the mechanical strength