Effects of hollow fiber packing fraction on gas transfer rate, blood pressure drop and blood flow characteristics of an intravascular oxygenator (IVOX) were evaluated by in vitro experiments. An IVOX module was prepared by packing hollow fiber membranes, of which packing fraction ranged from 0.056 to 0.338, in a pipe corresponding to vena cava. Overall mass transfer coefficients for oxygen and carbon dioxide were evaluated using water, and gas transfer rates through blood were estimated. Blood flow in the IVOX was evaluated by both pulse response method and electrode method. Blood flow was not uniform and depended on the hollow fiber packing fraction. Easy-flow channels between vascular wall and bundle of the hollow fibers were produced and stagnation of blood flow occurred in the bundle of the hollow fibers at the blood inlet. More uniform blood flow was obtained by optimizing the hollow fiber packing fraction. With respect to gas transfer rate, blood pressure drop and uniformity of the blood how the optimum hollow fiber packing fraction was approximately 0.28 when the outside diameter of the hollow fiber was 300 mum. Effect of inner diameter of the hollow fibers was also evaluated, demonstrating that reduction in the inner;diameter was an effective way to increase oxygen transfer rate.