TiOx nanoparticles prepared by a laser ablation of a TiO2 (rutile) target in a mixture gas flow of helium and oxygen were size-selected by a differential mobility analyzer, which classifies charged particles by differences in their electrical mobility. In typical ablation conditions, we used the second harmonic generation of Nd:YAG laser (wavelength 532 nm), of which the pulse energy and the repetition rate were 200 mJ/pulse and 10 Hz, respectively. The particles were observed with transmission electron microscopy (TEM) and Raman scattering spectroscopy after deposition onto carbon coated micro-grids and silicon substrates, respectively. The crystallinity of the particles classified at 10 run changed from amorphous to crystalline such as anatase and rutile as the annealing temperature increased from room temperature to 900 C, though spherical particles with a diameter of several tens of nanometers (droplets) prepared without annealing showed clear lattice fringes by an electron diffraction. The high-resolution TEM observations revealed that many kinds of Magneli phase titania were included in the prepared particles. (C) 2002 Elsevier Science B.V. All rights reserved.