In this work

spinel Fe

3

O

4

magnetic nano particles were synthesized by chemical co-precipitation method. The excess oleic acid was added during the initial stage of the reaction to prepare water-based ferrofluid under an alkaline condition. Subsequently

a novel and facile modified method were proposed by adding hydrochloric acid to adjust pH value or adding ethanol to successfully change the bilayercoated hydrophilic particles to hydrophobic ones. The coated particles were characterized by Fourier infrared spectrometer (FTIR)

thermogravimetry analyzer (TG)

transmission electron microscopy (TEM)and vibrating sample magnetometer (VSM). The results showed that the prepared water-based ferrofluid had the formation of inner layer chemical and outer layer physical adsorption by oleic acid and the particles'bilayer-coated structure was maintained by the acid modification through protonating the terminal carboxyl group

which had the maximum weight loss rate. The bilayer-coated Fe

3

O

4

particles were monodispersed in hexane indicated in TEM image. The stability of the ferroluid was much better than that of single coated particles because of the space steric effects'promotion caused by the bilayer-coated structure.