Core-shell magnetite/polyethyleneimine nanoparticles (Fe3O4/PEI) were synthesized by a precipitation method using polyethyleneimine (PEI) as a stabilizer. Au colloids-modified magnetic nanocomposites (Fe3O4/PEI/Au) were then prepared by a photochemical method in a HAuCl4 aqueous solution, using Fe3O4/PEI as the magnetic core. The samples were characterized by transmission electron microscope, X-ray diffraction and vibrating sample magnetometer. The results showed that Fe3O4/PEI nanoparticles with a mean size of 6.8 nm were quasi-superparamagnetic and the saturated magnetization is at about 37.4 A?m2?kg-1. The obtained Fe3O4/PEI/Au is irregular in shape, composed of a sub-micron aggregate of Fe3O4/PEI nanoparticles and some Au colloids with sizes ranging from 13 nm to 23 nm. Additionally, the quasi-superparamagnetic nanocomposites are magnetically susceptible and the saturated magnetization reaches 36.9 A?m2?kg-1. Therefore, Fe3O4/PEI/Au nanoparticles can be used in many areas including bioseparation and biosensors.Cited