Rare earth hydroxide nitrates are prepared from rare earth nitrates by hydrothermal treatment. Three crystal phases, M(OH) 3 (M1), M-2(OH)(5.14)(NO3)(0.86)center dot H2O (M2), and M4O(OH)(9)NO3 (M4) are identified. The crystal phase systematically changes from M1 to M4 through M2 with decreasing radius of rare earth cation. Morphology of nanocrystal depends on the crystal phase. M1 and M4 phases grow to be nanorod, and M2 to nanoplate. (C) 2017 The Ceramic Society of Japan. All rights reserved.