Single-walled carbon nanotubes are novel molecular-scale wires having excellent anti-adhesion properties with regard to platelets. On the other hand, chitosan is a partially de-acetylated derivative of chitin that has a critical role in cell attachment and growth. The aim of this study was to investigate how carbon nanotubes improve the blood biocompatibility of chitosan film. We prepared composite films with various concentrations of chitosan/carbon nanotubes (CS/CNTs) (1.3-6.3 wt%). The sample surfaces were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle measurements. The surface characterization revealed that the surface of the CS/CNTs composite film became more hydrophobic with increasing amounts of CNTs. Cell attachment tests using bovine aortic endothelial cells (BAECs) indicated that CS/ CNTs composite films retained their cell adhesion ability. The blood compatibility of the CS/CNTs composite films was evaluated using the blood platelet adhesion and activation tests in vitro. Platelet adhesion results confirmed that platelet adhesion and the formation of a platelet network were inhibited on composite films with higher concentrations of CNTs (5.1 wt%). Our experimental results show that the novel composite film containing CS/CNTs possesses two paradoxical characteristics, namely, good adherence of endothelial cells and minimum adherence and activation of platelets, making this film a promising antithrombogenic material for use in the biomedical field.