An outside blood flow membrane oxygenator with an optimum blood now pattern is developed by a trial and error method where large cost and long time are spent. We modeled the blood flow in the membrane oxygenator as the flow of low Reynolds number in a porous media and evaluated blood pressure drop and oxygen transfer rate using Darcy＇s law. Governing equations are solved by the finite element method. Theoretical blood now patterns agree well with visualized flow patterns obtained by X-ray computed tomography. Calculated values for blood pressure drop and oxygen transfer rate also agree with experimental data. Optimum structure parameters of the membrane oxygenator can hence be determined by simulation analysis.