The hollow fiber oxygenator is the most advanced one for the cardiopulmoanry bypass. They have two different types of the hollow fiber systems according to the way how the blood go through the fibers. One is inside blood flow type and the other outside type. In order to find out which is better to prevent blood cell destruction, we selected 40 valve replacing patients and divided them into 2 groups prospectively. In group I [n=20], inside blood flow type[BCM-7a], CO2 excretion is more effective than group II, that is partly because of the relative large surface area of the BCM-7. In group II [n=20], outside blood flow type [MAXIMAa], they have better quality to preserve platelet count. We also studied about several other items such as SaO2, Hemoglobin and RBC, WBC, fibrinogen, LDH, plasma hemoglobin, haptoglobulin and so on. But we cannot find any differences between two groups with any statistical meanings [p<0.05]. We conclude that both of two oxygenators are excellent in the aspects of gas exchange and blood cell preservation.