Heparin was covalently bonded on chemically inert PET substrate using a surface modification technique–surface interpenetrating network with the purpose of fabricating long-lasting biocompatible materials as vascular grafts. FTIR and XPS spectra confirmed the successful heparinization of PET (PET-Hep). The density of surface-immobilized heparin as quantified by a colorimetric method could reach 2.4 μg cm−2 (in the reported optimal range: 1.5–3.0 μg cm−2). The hemocompatibility of the heparin-immobilized PET was improved as evidenced by a platelet adhesion test: significantly less platelet adhesion on PET-Hep (11.60%) than on untreated PET (48.91%). An MTT assay indicated PET-Hep was nontoxic to human dermal fibroblast cells. After an initial 5.24% loss of heparin from PET-Hep in the first 14 h immersion in PBS buffer solution, no further leaching of heparin was found.