Porous magnesium hydroxide nanoplates were prepared directly from commercial bulk magnesium oxide crystals by a simple hydrothermal treatment. These thin plates would aggregate into large spherical particles. The platelike morphology was retained after calcination, and porous magnesium oxide nanoplates were obtained. These plates have a wormhole-like porous structure with high surface area. The obtained materials exhibit bimodal pore size distributions in the mesoporous domain. The aggregation of the nanoplates gives rise to large mesopores with a size of about 36 rim. In addition, each plate has small wormhole mesopores with a size of about 3.7 nm. The growth of magnesium hydroxide nanoplates occurred through a dissolution-recrystallization process. X-ray diffraction and electron diffraction, transmission electron microscopy, scanning electron microscopy, and nitrogen sorption have been employed to characterize these nanoplates. Such porous nanoplates with high surface area and high crystallinity have many promising applications. Moreover, bismuth oxide nanoplates were also produced following a similar method. The formation mechanism of such mesostructures without the use of a template is also discussed.