Abstract: The nonlinear oscillation can be described by FlynnEquation when considering the compressibility of inner gas of bubbles. Basedon the model the displacement/the time relation and phase-trajectory ofvibrating bubbles of different initial radii are simulated numerically,which shows that the movements of bubbles depend strongly on the initialconditions in the same sound field. When the driving frequency of 26.5kHzand intensity of 1.35atm is adopted, the bubbles oscillate forcedly withinitial radii are smaller than 1\mu m, while oscillate quasi-intrinsicallywith initial radii larger than 200\mu m. The two kinds of bubbles are notcavitation ones. The changes of smaller bubbles are more intense to thesound intensity. If the sound intensity increases, a restricted range ofbubbles smaller than 1\mu m can become cavitation ones. Furthermore, thesmaller the initial radius is, the larger the increase of maximumdisplacement is. In addition, the driving frequency also affects themovement of bubbles. With the increase of driving frequency, the initialradii range of cavitation bubbles decrease, and the oscillation intensityreduces. The results present that there are three types of bubbles in theultrasonic field: big bubbles oscillated quasi-intrinsically, cavitationbubbles and micro-bubbles oscillated forcedly at the driving frequency. Thesize range of cavitation bubbles is influenced jointly by driving frequencyand pressure amplitude. At the same time, we researched the bubbles field intap water by high speed photography when the ultrasonic transducer isworking. The results show that there is a mixed field, where many bubbleswith different radius are vibrating and moving. Not only cavitation bubblesbut also bubbles with the millimeter scales coexist in the radiation fieldof ultrasonic transducer. The movements of bubbles can affect the intensityof cavitation directly. There is a complex physical field when cavitationcomes into being in the liquid, where bubbles are vibrating, colliding andcombining.