Heparin is a well-known anti-thrombogenic polymer which is widely used in surgery. In this study, heparin sodium was used to modify the surface of poly(L-lactic acid) (PLLA) in order to enhance its biocompatibility. The properties of a modified PLLA surface and the control sample were investigated by water contact angle measurement and electron spectroscopy for chemical analysis (ESCA). The former indicated that the hydrophilicity of the modified PLLA film was improved with the immersion time and the concentration of heparin sodium in the solution. The latter suggested that the modified PLLA film is enriched with nitrogen atoms. The biocompatibility of PLLA film is altered and in turn affects the anticoagulant activity. The number of platelets deposited on the modified film was greatly decreased. These data suggest that heparin can improve the in vitro blood compatibility of the PLLA film.