Magnesium hydroxide nanostructures have been synthesized by the reaction of magnesium acetate with sodium hydroxide via sonochemical method. Reaction conditions such as the Mg(2+) concentration, aging time and the ultrasonic device power show important roles in the size, morphology and growth process of the final products. The magnesium oxide nanoparticles have been prepared by calcination of magnesium hydroxide nanostructures at 400 degrees C. The magnesium hydroxide and magnesium oxide nanostructures were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), thermal gravimetric (TG) and differential thermal analyses (DTA).