In this paper we present Flow Rate Profiler (FRP), an instrument for measuring the blood velocity by means of ultrasound-based techniques. The velocity is directly related to the shear rate, which is in turn proportional to the shear stress, a parameter expressing the pressure exerted by the blood on the vessel walls. The knowledge of this value is important in medicine to establish the state of the vessels, directly related to vascular diseases. FRP provides a non-invasive measure of the blood velocity by exploiting the red corpuscles property of diffusing ultrasound waves: in practice blood velocity is determined by a cross-correlation technique, which analyses the time shift between correlated subsequent echo waves, instead of frequency shift characteristic of the Doppler technique. The acquired data are then processed on a personal computer by means of mathematical techniques based on the evaluation of the correlation function, giving a reconstructed velocity profile and showing a good adherence with experimental data, since the average error is nearly the 10%. The reconstructed profile is displayed to the operator, who can follow the vessel status in real time. A few comparisons between the reconstructed and the experimental profiles are also presented, together with a study on a small set of patients suffering from artery hypertension.