The patient undergoing open-heart surgery depends on hypothermia and cardiopulmonary bypass (CPB) to maintain organ perfusion during cardiac arrest. Increased blood viscosity during hypothermic CPB might be life-threatening, so hemodilution is imperative. Fourteen open-heart patients of ASA class II-III were included in this study. Pre- and intra-CPB viscosity changes were observed. Before CPB, 6 ml blood sample was drawn out from open-heart patients after systemic heparinization and another 6 ml blood sample was drawn out from the oxygenator after CPB was established. The hematocrit and relative viscosity of each sample in different temperatures were measured. The result revealed that as the temperature decreases, the viscosity of each sample increases apparently. As compared to the pre-CPB blood sample, the hematocrit of blood obtained intra-CPB decreases from 36.81 to 27.04, (p < 0.001), and the viscosity also decreases at all different temperatures (p < 0.001). Blood viscosity obtained at 37 degrees C pre-bypass is not statistically different from the sample at 25 degrees C during bypass. Obviously, increased viscosity due to hypothermia is buffered by hemodilution after using large amount of fluid as priming solution during CPB. Therefore, the hematocrit and viscosity remain within physiologic ranges.