In this study, we examined the preparation of chitosan-coated magnetic nanoparticle (MNP) (chitosan- MNP). The Fe3O4–chitosan nanoparticles with core-shell structure have been prepared by crosslinking method Fe3O4 nanoparticles were firstly prepared by co-precipitation then chitosan was added to coat on the surface of the Fe3O4 nanoparticles by physical absorption. X-ray diffraction pattern indicated that the magnetic nanoparticles were pure Fe3O4 with a cubic inverse spinel structure. The modification using chitosan did not result in a phase change. The binding of chitosan to the Fe3O4 nanoparticles was also demonstrated by the measurement of Fourier transform infrared spectra. DLS showed Particle size distribution was 49-60nm.We reported a new co-precipitation method to prepare magnetite nanoparticles with negative surface charge and narrow size (49-60) distribution and was coated with chitosan.