Abstract: Antithrombogenic synthetic elastomer having repeat units comprising portion I which is a soft segment unit containing a polyether and portion II which is a hard segment unit: ##STR1## R is a straight-chain or branched-chain alkylene group containing 2 to 4 carbon atoms, R' is amide a urethane, an urea, n ranges up to 180, m is an integer of 1 to 20, l is an integer of 1 to 10, said polymer having a microdomain structure composed of soft and hard segments, the domains of which have an average size of 10 to 20 nm, and a molecular weight of about 80,000 to about 500,000.The polymer shows little adhesion of blood platelets thereto, little deformation of adhering blood platelets and an excellent antithrombogenic property. This polymer has excellent mechanical properties as an elastomer.