Abstract: The currently used methods of ultrasonic blood flow measurements are studied in this paper.Their inherent characteristics and relations are pointed out. Pulsed Doppler system and autocorrelation method are essentially the same narrowband estimators with the presumption of narrowband signals.Since the presumption is not fulfilled by the real ultrasonic signals, they give the results with low velocity resolution and probably the aliased spectrum. The wider the frequency band of the signals are, the worse the results will be. Time domain crosscorrelation technique, wideband maximum likelihood estimation and 2D Fourier transform method are three wideband techniques. The high velocity resolution and low probability of aliasing can be achieved by these wideband techniques. Moreover, the better estimations come out with wider signal frequency band and the more pulse echoes.