In this paper, we developed a model that uses pressure-flow monitoring information in the oxygenator to estimate viscosity of human blood. The comparison between estimated viscosity (ηe) and measured viscosity (η) was assessed in 16 patients who underwent cardiac surgery using mild hypothermia cardiopulmonary bypass (CPB). After initiation of CPB, ηe was recorded at three periods: post-establishment of total CPB, post-aortic cross-clamp, and post-declamp. During the same period, blood samples were collected from the circuit and η was measured with a torsional oscillation viscometer. The ηe was plotted as a function of η and the systematic errors and compatibility between two methods were assessed using Bland-Altman analysis. The parameters ηe and η were very strongly correlated at all points (R(2)=0.9616, p<;0.001). The Bland-Altman analysis revealed a mean bias of -0.001 mPas, a standard deviation of 0.03 mPas, limits of agreement of -0.06 mPas to 0.06 mPas, and a percent error of 3.3%. There was no fixed bias or proportion bias for the viscosity. As this method estimates blood viscosity with good precision during CPB continuously, it may be helpful for clinical perfusion management.