Over the past decade, much attention has been paid to anti-thrombotic materials applied in artificial organs. Surface modification has shown potential to improve the anti-coagulation of blood-contacting biomedical devices and materials w1-3x. Our in vitro study of Ti-O thin films has recently shown that Ti-O thin films possess superior blood compatibility to low temperature isotropic pyrolytic carbon (LTI-carbon), w1x. In this work, we have focussed our attention onto the in vivo evaluation of Ti-O thin films fabricated by plasma immersion ion implantation (PIII). The samples of titanium coated by Ti-O thin films were implanted in dogs'hearts for 1 month. The results of the implantation showed that no thrombus was found on the surfaces of the Ti-O thin film, although the coagulation occurred on the surfaces of LTI-carbon.