Polymeric materials have been attracted the attention of researchers in various R&D applications. In this research, attempts were made to evaluate the changes in blood compatibility of polyethylene terephthalate (PET) by grafting acrylic acid (AAc) and immobilizing heparin by employing two-step plasma treatment. The PET surface was modified by using a novel method named “two-step plasma treatments” (TSPT). While first plasma is creating active sites for grafting, the second plasma polymerizing pre-adsorbed reactive monomer onto the surface of films. Finally, heparin immobilization was performed in the presence of 1-ethyl-3-(dimethylaminopropyl) carbodiimide. All films were characterized by attenuated total reflection Fourier transformed infrared (ATR-FTIR) spectroscopy and scanning electron microscopy (SEM). The surface hydrophilicity of films was studied by water contact angle test and blood compatibility evaluated by Lactate dehydrogenase method (LDH Test). In vitro studies based on LDH results demonstrared that platelet adhesion onto the modified surfaces with heparin was drastically reduced in comparison with unmodified PET.