Nanocrystalline magnetite particles have been prepared by hydrothermal reaction of ferrous sulfate (FeSO4), sodium hydroxide and sodium thiosulfate (Na2S2O3) at 140 degreesC. X-ray powder diffraction pattern indicates that the product is single-phase magnetite in an inverse cubic spinel structure. Its composition is determined by traditional chemical analysis combined with X-ray photoelectron spectroscopy. Transmission electron microscopy images show that the magnetite particles are homogenous in the shape of quasi-sphere polyhedron with average diameter of 50 nm.