Continuous M40J carbon fiber reinforced aluminum-magnesium alloy composite wires have been fabricated using the ultrasonic infiltration method. However, the factors controlling the infiltration of the molten alloys into the bundle of the carbon fibers have not been clarified. The infiltration phenomenon was examined especially from the viewpoint of acoustic cavitation. It was found that the infiltratability of the alloys was proportional to the maximum intensity of the acoustic cavitation. Both the infiltratability and the intensity were enhanced by the addition of surfactant elements into the molten aluminum. Thus, a decrease in surface tension will cause an increase in the generation of acoustic cavitation which will lead to molten aluminum infiltrating into the bundle of carbon fibers. Therefore, the generation of the acoustic cavitation will be the one of the factors controlling the infiltration during the fabricating process of these kinds of composites using ultrasonic vibration.