Pure aluminum ingot (99.8 wt%) was melted to prepared chilled samples in this study. These samples were then removed to polish their surfaces and put in an ultrasonic cleaner filled with tap water. The polished surface would gradually show foggy marks after being subjected to a period of treating time. Oxide films, if entrapped, would crack, erode and detach from the chilled sample forming foggy marks on the polished surface. The sample then removed to measure oxygen and aluminum concentrations varied along the transition layer between the oxide film and aluminum matrix. Part of chilled samples was melted in a muffle furnace and subjected to different holding time. As the holding time increased, the transition layer between the oxide film and the matrix was increased and composed of different constituents varying from the Al matrix to the oxide film (mainly gamma-Al2O3). This transition layer also showed different hardness measured by a nano-hardness tester. The morphologies of cracked oxide film and the eroded oxide particles were affected by the holding time after melted, and small amounts of silicon in the pure aluminum.