In this paper a study on the facts affecting blood compatibility of titanium oxide films synthesized by ion beam assisted deposition is presented, and the mechanism of blood compatibility is discussed. We think that the blood compatibility is determined by surface energy and work function of the film. The protein adsorption is determined by surface energy. An area with a range of values of polar and dispersion components of surface energies, where good protein adsorption property will exhibit, was defined. That is, more human serum albumin (HSA) and less human fibrinogen (HFG) will be adsorbed in this area. Work function is also an important factor to influence blood compatibility. Low work function can inhibit the decomposition of HFG and improve thromboresistance.