Ultrasonic backscattering of porcine whole blood (hematocrit at 40%) was measured by a 10-MHz catheter mounted transducer in a mock flow loop at different flow rates controlled by a roller pump and a pulsatile pump. The amplitude of the oscillatory flow was fixed at 40 cm/s and the baseline velocity was set at three different levels, 0, 20, and 40 cm/s, to change the shear rate range. As the flow rate increases from 0 to 40 cm/s to 20 to 60 cm/s, the peak of cyclic variation of the backscattering signal leads more of the peak of the mean flow velocity. At 40 to 80 cm/s, the magnitude of cyclic variation drops from 4 dB to 1.5 dB. It was found that at the two lower flow rate ranges the backscattering signal increases with shear rate if the mean shear rate is below 150 l/s. When the shear rate further increases, the backscattering signal decreases. The wave form of the cyclic variation is also altered as the baseline shear rate changes. Results show that, in simple oscillatory flow, the amplitude and the timing of the peak of ultrasonic backscattering from whole blood are affected mainly by the shear rate which mediates red cell aggregation.