A thick Al2O3/aluminum (Al) structure has been fabricated by oxidation of Al with 68wt% and 98wt% nitric acid (HNO3) aqueous solutions at room temperature. Measurements of the Al2O3 thickness vs. the oxidation time show that reaction and diffusion are the rate-determining steps for oxidation with 68wt% and 98wt% HNO3 solutions, respectively. Observation of transmission electron micrographs shows that the Al2O3 layer formed with 68wt% HNO3 has a structure with cylindrically shaped pores vertically aligned from the Al2O3 surface to the Al2O3/Al interface. Due to the porous structure, diffusion of HNO3 proceeds easily, resulting in the reaction-limited oxidation mechanism. In this case, the Al2O3/Al structure is considerably rough. The Al2O3 layer formed with 98wt% HNO3 solutions, on the other hand, possesses a denser structure without pores, and the Al2O3/Al interface is much smoother, but the thickness of the Al2O3 layer formed on crystalline Al regions is much smaller than that on amorphous Al regions. Due to the relatively uniform Al2O3 thickness, the leakage current density flowing through the Al2O3 layer formed with 68wt% HNO3 is lower than that formed with 98wt% HNO3.