Magnetite nanoparticles have been synthesized by thermal decomposition of hematite (oe-Fe203) powder in the presence of high boiling point solvent. The mixture of hematite and 1-octadecene solvent was heated and stirred in nitrogen gas at the temperature of 320 °C for the desired time (~2 to 28 hrs). The influence of the reaction time on transformation process was analyzed with X-ray diffraction (XRD), M5ssbauer spectroscopy (MS), and magnetic measurements. XRD patterns show that the phase of intermediate was composed of spinel phase and corundum phase (a-Fe203). The 57Fe Mossbauer spectra show that the spinel phase originated from the magnetite particles. The structure transformation proportion of hematite to magnetite strongly depends on reaction times. After reflux for 28 hrs the hematite-magnetite transformation was complete. The mean crystallite size of pure phase of magnetite particles is about 40 nm. The saturation magnetization increases with the reaction time, which corresponds to an increase of concentration of magnetite in the samples. A pronounced feature of the Hc and GrlGs observed in samples is the steplike change which appears at 125 K and is characteristic of the Verwey transition. The hyperfine parameters of Mossbauer spectrum measured at low temperature also indicate that the Verwey phase transition occurs. In other words, the Verwey transition is an indication that the magnetite particles exactly grew up in the synthesized compounds. This thermal decomposition process provided a method to prepare pure magnetite as well as magnetite/hematite nanocomposites useful for various magnetic applications.