Nanocrystalline barium titanate (BaTiO3, BT) was synthesized hydrothermally at 220 degrees C by reacting barium hydroxide with titanium dioxide. The resulting BT nanopowders were characterized by X-ray diffraction (XRD), selected area electron diffraction (SAED), Raman spectrum, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HR-TEM). The BT nanopowders have a cubic phase as revealed by the XRD and SAED results, whereas the Raman spectrum indicates that tetragonal phase BT exists in the produced nanopowders but is not the dominant phase because of the weak characteristic peak of the tetragonal structure at 305 cm(-1). In the SEM and TEM images, many coarse-faceted cubic nanoparticles and a small amount of spherical particles are observed, indicating a developed bimodal size distribution of the BT powders. The variations of electron diffraction contrast across these particles in the TEM images indicate the presence of high strain in the BT nanoparticles, which is probably caused by the lattice defects like OH- ions and their compensation by cation vacancies. The HR-TEM image of a BT nanoparticle with a size of 24 nm and a spherical morphology demonstrates a uniform and perfect crystal structure. The surrounding edges of the particle are very smooth and no surface steps are observed. However, a terrace-ledge-kink (TLK) surface structure was frequently observed at the edges of the BT nanoparticles with rough surface morphology, and in most cases the terrace and ledge lie on the {100} planes. The observed TLK surface structure can be well interpreted by the theory of periodic bond chains. Small nucleated and triangular BT islands with three to four atomic layer thickness, and their outside surfaces faceted as (100) and (010) planes, are also observed in these particles. The rarely seen {110} surface in the BT nanoparticles was found to be reconstructed so that the surface was composed of corners bound by {100} minifaces like the triangular small islands. Microstructural defects such as antiphase boundaries were also observed near the edge of a BT nanoparticle, which were formed by the intersection of two crystalline parts with displacement deviation from each other by 1/2d(110), as revealed by the HR-TEM images.