Ultrafine BaTiO3 nanoparticles and their highly stable sols are prepared by a novel and rapid route. In this method, the formation mechanism that lies between the chemical precipitation and the sol-gel process is proposed. The BaTiO3 nanocrystal sols are synthesized in as fast as 15min in an air atmosphere. Dynamic light scattering analysis and the observation of the Tyndall effect confirm the existence of crystalline nanoparticles in these sols. After careful separation, nanocrystalline BaTiO3 powders with an average particle size as small as 2.8nm are obtained. These particles have perovskite phase structures as determined by X-ray diffraction and selected-area electron-diffraction analysis. Fourier transform infrared spectroscopy (FT-IR) and thermal analysis are used to detect the characteristic functional groups of the solvents on the particles to reveal the formation mechanism. Uniform BaTiO3 nanocrystal films with high dielectric constants, low dielectric losses, and paraelectric behavior are prepared through solvent evaporation of the nanocrystal sols, providing a new low-temperature route for the fabrication of perovskite thin films.