In this study, an aluminum oxide layer for sealing treatment of nano-diamond powder was synthesized byanodizing under constant current. The produced pore size and oxide thickness were investigated using scanning electronmicroscopy. The pore size increased as the treatment time increased, current density increased, sulfuric acid concentra-tion decreased, which is different from the results under constant voltage, due to a dissolution of the oxide layers. Theoxide layer thickness by the anodizing increased as temperature, time, and current density increased. The results of thisstudy can be applied to optimize the sealing treatment process of nano-diamond particles of 4-10 nm to enhance theresistances of corrosion and wear of the matrix.