The microstructure of BaTiO3 (BT) nanocrystals was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), selected area electron diffraction (SAED), and (high-resolution) transmission electron microscopy [(HR)TEM]. The BT nanocrystals have a cubic perovskite structure as revealed by XRD and SAED. They were prepared by a hydrothermal method using different starting materials: (i) Using as-prepared titanium hydroxide and barium hydroxide as starting materials, BT nanocrystals were obtained with a narrow size distribution and an average grain size of 65 nm, exhibiting a spherical particle morphology and smooth surfaces. These nanocrystals showed almost uniform contrast in TEM. HRTEM lattice fringe images indicate a rather uniform, perfect crystal structure. (ii) Using barium hydroxide and titanium dioxide (anatase) as starting materials, many coarse-faceted cubic nanocrystals and a small amount of spherical particle were observed, indicating a developed bimodal size distribution. Diffraction contrast variations across these particles indicate the presence of high strain in these BT particles. The strain is probably caused by lattice defects like OH- ions and their compensation by cation vacancies. (HR)TEM images demonstrated that (100) and (110) surface facets are present on these particles. In addition, they revealed the presence of very small nucleated BT islands at the edges of the faceted particles. (c) 2005 Elsevier B.V. All rights reserved.