Abstract: Magnetite nanoparticles (Fe3O4) were synthesized by the solvothermal method at 160℃，using FeCl3·6H2O and ethylene glycol as reactants. X-ray diffraction，field emission scanning electron microscopy and vibrating sample magnetometer were used to characterize the magnetite nanoparticles. The results indicated that the magnetite nanoparticles were of inverse spinel structure. The average particle sizes of 16 h and 24 h reaction were 145 nm and 495 nm respectively. The particles did not make any agglomeration. They showed outstanding monodisperse property. The influence of reaction time on the particle size of the products was studied. The saturation magnetization of reaction 24 h was 79.3 emu/g. The above particles showed excellent magnetic property.