Abstract: The precursors，basic magnesium borate whiskers［MgBO2（OH）］were hydrothermally synthesized with magnesium chloride hexahydrate（MgCl2·6H2O） and boric acid（H3BO3） as the raw materials.Magnesium borate（Mg2B2O5） whiskers were obtained by calcination of the precursors.The compositions and morphologies of MgBO2（OH） and Mg2B2O5 were characterized by XRD，SEM，and TG-DTA. The influence of the reactant concentration on the composition and morphology of MgBO2（OH） was mainly investigated.The forming process of the precursors was also analyzed.Results showed that，the precursors′ prepa- ration was directly related to the concentration of the raw materials.When the concentration of Mg2+ was 2.0 mol/L，and the concentration of H3BO3 was 0.75 mol/L，MgBO2（OH） whiskers with good morphology could be synthesized.The subsequent thermal conversion of the MgBO2（OH） whiskers at a relatively low temperature of 650 ℃ for 3 h led to pure phase of  Mg2B2O5 whiskers，with the diameter of 40~70 nm，the length of 500~1 500 nm，and the aspect ratio of 10~20.

Key words: hydrothermal method, controllable synthesis, basic magnesium borate, magnesium borate, whisker