Boehmite (γ-AlOOH) nanocrystals with varying morphologies were synthesized by hydrothermal treatment of the freshly precipitated Al(OH)₃ gel at 240 °C for 16 h in different solutions. Under acidic conditions (initial pH 4.0), boehmite nanorods with an aspect ratio of about 2 and 4, respectively, were formed in the presence of nitrate and chloride, and boehmite nanowires with an aspect ratio of about 80 were formed in the case of sulfate. Under alkaline conditions (initial pH 10.5), boehmite nanoplates with a size of about 100 nm were produced, irrespective of the anion types. Further analyses indicated that the adsorption of the anions on a boehmite surface may be responsible for the shape formation of boehmite. Under acidic conditions the adsorption order was as follows: nitrate < chloride < sulfate, in agreement with the preferential growth tendency order: nitrate < chloride < sulfate, while little anions can be adsorbed under alkaline conditions. The selective adsorption of anions on the (010) and (001) facets of boehmite is suggested to be responsible for the preferential growth of boehmite along [100] direction.