Crystal growth behaviors of boehmite particles with octanoic acid and sodium octanoate under supercritical hydrothermal condition were investigated. It was confirmed that adding carboxylic acid is effective in synthesizing hexagonal plate particles with high aspect ratios, while rhombic plate particles were obtained with nitric acid at similar pH condition. To investigate the modification under basic condition, sodium octanoate was used as the additive instead of octanoic acid. The increase in concentration of octanoic acid brought about high aspect ratio boehmite particles, while adding sodium octanoate resulted in low aspect ratio boehmite. Since the particles synthesized at more acidic condition were densely covered by octanoic acid, it is considered that the surface modification by octanoic acid was facilitated at such acidic condition. These results indicate that pH in the reaction field is one of the important factors to obtain the carboxylic acid modified boehmite particles with high aspect ratios, as well as other factors such as the carboxylic acid concentration and treatment time. (C) 2016 Elsevier B.V. All rights reserved.