Boron nitride nanocrystals with high crystalline perfection and high yield have been successfully prepared by a hydrothermal synthesis route. The experimental results show that the addition of N2H4 . H2O and NH4Cl into the reaction mixture significantly improved the perfection and sample yield. Furthermore, the results of powder X-ray diffraction, fourier transformation infrared spectroscopy, transmission electron microscopy, selective area electron diffraction and high-resolution transmission electron microscope analysis proved that the major phases in the samples were orthorhombic boron nitride and cubic boron nitride. (C) 2004 Elsevier B.V. All rights reserved.