In our previous study [J. Appl. Phys. 106, 023524 (2009)], it was confirmed that aluminum (Al) film was transformed into transparent film by hydrothermal-treating in ultrapure water. In addition, transmittance of this specimen became larger than that of the glass substrate itself. This is useful property for anti-reflective coating. In this study, transmission electron microscopy has been performed to observe structural change during boiling 100 nm-thick Al film. After boiling, the film expands 4 or 5 times in thickness. However, the structure is not homogeneous along surface normal of the substrate. There is density gradient structure corresponding to boehmite crystal growth from embryo to crystal sheet less than 10 nm thickness. This kind of structure is thought to be one of the structural reasons for low reflectance.