Physical and chemical properties of the magnetic nanoparticles coated with oleate have been investigated by means of transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Mossbauer spectroscopy, superconducting quantum interference device (SQUID) magnetometry, infrared spectra (IR) and the contact angle device. The results show that doped Al ions in Fe3O4 nanoparticles are located on the octahedral sites of the spinel structure. Oleate is coated on the magnetic nanoparticles with two layers by chemical absorbing, the outer layer can be washed away. The wetting of the surface of magnetic nanoparticles coated with monolayer has been changed from hydrophilicity to hydrophobicity, and the nanoparticles can be dispersed very well in some organic solutions. Physical and chemical properties of the magnetic nanoparticles coated with oleate have been investigated by means of transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Mossbauer spectroscopy, superconducting quantum interference device (SQUID) magnetometry, infrared spectra (IR) angle device. The results show that doped Al ions in Fe3O4 nanoparticles are located on the octahedral sites of the spinel structure. The wetting of the surface of magnetic nanoparticles coated with monolayer has been changed from hydrophilicity to hydrophobicity, and the nanoparticles can be dispersed very well in some organic solutions.