Heparin (an anti-thrombin) was immobilized onto microporous poly (vinylidene fluoride) (PVDF) membranes that were prepared either by dry or immersion precipitation method. Immobilization of heparin was carried out by a procedure consisted of two steps. First, poly (acrylic acid) (PAA) was grafted on PVDF membranes with various surface porosities by plasma-induced polymerization. Then, heparin was covalently bonded to PAA with the aid of 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride (EDAC). The highest attainable grafting yield of PAA was 0.68 mg/cm2. For heparin immobilization, it was found that the preparation parameters such as, EDAC activation time, concentration of reactants, and pH affected significantly the amount of immobilized heparin. The maximal value was 9.68 μg/cm2. Furthermore, blood compatibility tests were carried out on membranes with and without heparin by platelet adhesion method. The results indicated that immobilized-heparin could effectively inhibit platelet adhesion on the membranes.