Monodispersed Fe3O4 magnetite nanoparticles were successfully synthesized via a simple solvothermal method, in which Fe(NO3)3•9H2O was used as the starting materials, KOH as the mineralizer, and ethylene glycol (en) as the solvent. X-ray diffraction (XRD) and selected area electron diffraction (SAED) were employed to characterize the phase composition, transmission electron microscope (TEM) to observe the morphology and the particle size, and physical property measurement system (PPMS) to investigate the magnetic property of the synthesized powders, respectively. The synthesized Fe3O4 magnetite nanoparticles are of 50-100nm in size, and of notable ferromagnetic property. The saturation magnetization, remanent magnetization, and coercive field are 68.8emu•g-1, 12.9emu•g-1, 138.5Oe, respectively. Based on the experimental resuts, the formation mechanism and the well monodispersed reason of the solvothersized Fe3O4 nanoparticles are discussed.