An “active” ultra sound Doppler (USD) method was newly developed for the direct, easy and quantitative discrimination of emboli from micro bubbles in blood flow. This method is based on the measurement of the sound axial velocity of the two induced by using acoustic radiation force of ultra sound wave. This force depends on the difference of characteristic acoustic impedances between ambient fluid and emboli or micro bubbles. In the case of a micro bubble, the change of the moving velocity is larger because of a much larger difference between the characteristic acoustic impedances of the two. In order to prove the availability of active USD method, an in-vitro experiment on discrimination of emboli from micro bubbles was performed in a flow modeled after human's middle cerebral arterial flow. As model emboli, blood clot and alginate gel particle were used, and as working fluid, calcium chlorite aqueous solution and bovine blood. In the experiment, model emboli and micro bubbles were irradiated from the downstream side by ultra sound wave from commercial ultrasound diagnostic equipment being used at clinical sites. It was found that there were obvious differences of moving acceleration in the sound axial direction between micro bubbles and model emboli in the modeled blood flow which was caused by the acoustic radiation force, and that this “active” USD method had high availability for a medical diagnosis system to directly, easily and quantitatively discriminate emboli from micro bubbles flowing in human vessel.