In this paper, uniformed and narrow size distributional Fe3O4 nanoparticles were synthesized by thermal decomposition method. After that these nanoparticles were modified with polyethylene glycol monomethyl ether by thiol-ene method. They were characterized by XRD, TEM, FT-IR, Raman, TG, VSM and UV. It was showed that this method was simple, the reaction conditions was mild and the operating conditions was easy. These functionalized Fe3O4 nanoparticles were not only well dispersed in water and PBS, but also showed superparamagnetic property.