Single crystal Al (100) was exposed to 5 eV atomic oxygen beam. The sample was maintained at a temperature of 220°C and the total atomic oxygen fluence was 8×1019 atom.cm-2. We have characterized the resulting oxide and interface structures by cross-sectional (scanning) transmission electron microscopy ((S)TEM) and scanning electron microscope(SEM). Our TEM results show that an amorphous aluminum oxide layer with ∼6 nm thickness formed on the aluminum crystal, and a rough alumina/Al(100) interface forms. For a systematic study of the evolution of the oxide, a unique Physical Sciences, Inc. Pitt FASTTM AO laser detonation atomic oxygen source in a UHV chamber is employed. The system is equipped with a Maxtek RQCMTM system, a research quartz crystal microbalance (QCM) with a dual-sensor head, to dynamically measure the mass change of an aluminum film coated on the sensor crystal during exposure to atomic oxygen. The Al film initially experiences mass loss, and then parabolic mass gain. To observe the structural evolution of the oxide, a very thin Al (100) single crystal was exposed inside the AO source, characterized by SEM and TEM. The surface morphology changed from flat to rough after 5.5 minutes of exposure. This surface roughening could be related to the initial mass loss measured by QCM.