A series of multicrystal and monocrystal samples of yttrium orthoborate YBO3:Eu with different morphologies and sizes were prepared by a simple hydrothermal (HT) method at a low temperature of 260 degrees C for 3h, in the absence of any surfactant and template, without milling and post calcination. Samples are characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM). The morphologies and sizes of YBO3:Eu, most of which are prepared for the first time, include polycrystalline nano-sheet; monocrystalline nano-sheet, nano-particle, submicro-sheet, submicro-rod and submicro-tube; micro-flower and micro-downy ball, which are assembled by orderly nano-sheets. At the same time, multifarious clear and high-quality selected-area electron diffraction (SAED) patterns including multicrystal diffraction rings and monocrystal diffraction spots were obtained for the first time, which provide abundant stuff to study the crystal structure of YBO3. However, all the innermost diffraction rings or spots of SAED patterns cannot be indexed by the currently accepted hexagonal crystal structure of YBO3, whose space group is P6(3)/m and lattice parameters are a=3.778 angstrom, c=8.810 angstrom. On the other hand, the whole diffraction pattern can be successfully indexed by another hexagonal structure, whose space group is P (6) over bar c2 and lattice parameters are a=6.454 angstrom, c=8.810 angstrom. In addition, this structure is also supported by the XRD results. As a result, the crystal structure of YBO3 is clarified. (c) 2009 Elsevier B.V. All rights reserved.