Oxide films form readily when aluminum alloy castings are melted and/or poured. They could be primary or secondary types of oxide films. The former type is inherited from the ingot and has existed in aluminum alloy casting for a long period of time. During the filling of the mold cavity, the free unstable surface of the molten metal causes a secondary oxide film to exist on the aluminum alloy castings. The oxide films are usually rich in oxygen and are difficult to observe by optical micrographs. This paper presents a simple but powerful method to observe the shape and size of oxide films on the aluminum matrix. During ultrasonic-vibration treatment, cavitation bubbles nucleated, grew and collapsed, generating micro-jets on the surface of sample. The water micro-jets then had an impact on the oxide film initiating micro-cracks. The cracks grew or became linked together which caused fractures in the oxide film. Small or tiny oxide particles detached from the oxide film to erode the surface of the treated sample. This eroded surface would show as a foggy mark via visual or optical observation. A series of photographs were made and are shown to illustrate the cavitation erosion process of oxide film on the surface of an aluminum sample. In addition, the presented method was shown to be useful in the diagnosis of oxide films that formed on aluminum and magnesium alloys, including the ingots, casting or wrought products.