An “active” ultrasound Doppler method was newly developed in order to directly and easily discriminate emboli from micro bubbles flowing in blood vessel. The principle of this method is based on measuring the induced velocities of an embolus and a micro bubble by acoustic radiation force of ultrasound wave. This force is mainly influenced by the difference in characteristic acoustic impedances between emboli or bubbles and ambient fluid. In order to prove the availability of this new discrimination method, micro bubbles, model emboli and the other particles were irradiated by ultrasound wave from a commercial ultrasound diagnostic equipment. Every motion was observed by a high speed video camera and their velocities along sound axis were measured. As a result, it was found that micro bubbles were strongly moved with a velocity along the axis in steady flow while model emboli were hardly moved, and that this “active” ultrasound Doppler method had high availability for a medical diagnosis system to discriminate emboli from micro bubbles flowing in human vessel.