Although blood viscosity has attracted much attention for its effect on hemodynamic parameters related to atherosclerosis, quantitative method for evaluating blood viscosity in vivo is not currently established. The purpose of this study was to verify the feasibility of blood viscosity estimation by a two-dimensional ultrasonic-measurement-integrated (2D-UMI) analysis system that computes an intravascular blood flow field by feeding back an ultrasonic measurement data to a numerical simulation. A method to estimate blood viscosity was proposed by reproducing the flow field of an analysis object in the feedback domain of ultrasonic Doppler velocity in a 2D-UMI blood flow analysis system, and evaluating the variation of the Doppler velocity caused by the analysis viscosity in the nonfeedback domain at the downstream side. In a numerical experiment, a viscosity estimation was performed for numerical solutions of sinusoidal oscillating flows analyzed as a blood flow model in a human common carotid artery at four different types of blood viscosities. The estimation viscosities were made to correspond to those of all analysis objects by giving proper conditions on the feedback gain and feedback domain to optimize the accuracy of the 2D-UMI blood flow analysis. In conclusion, the feasibility of blood viscosity estimation by 2D-UMI analysis was established. Simultaneous measurement of the in vivo blood viscosity and flow field can be easily performed in many clinical cases by its widespread use at clinical sites, thereby clarifying the relationship between hemodynamics and vascular pathology for various blood flow fields.