Abstract: In order to save acid solution step，well crystalline and lower surface polarity magnesium hydroxide was prepared.The magnesium hydroxide was obtained with direct conversion hydrothermal，and through taking activated brucite as raw material and induced by sodium p-toluene sulfonate.The optimum reaction conditions were selected by the single factor test of the hydrothermal time，temperature，and NaOH concentration.The optimum reaction conditions were determined as follows: amount-of-substance ratio of sodium p-toluene sulfonate to brucite was 1∶20，filling degree was 70%，hydrothermal time was 12 h，hydrothermal temperature was 200 ℃，and concentration of NaOH solution was 5 mol/L.And the average grain size of nano-sized magnesium hydroxide was calculated as 14.6 nm by Scherrer formula.

Key words: brucite, sodium p-toluene sulfonate, inducer, hydrothermal