Magnetic nanoparticles that preferred for biomedical applications are required to be biocompatible, nanosized and superparamagnetic. In this research, ultra-small superparamagnetic Fe3O4 nanoparticles and novel superparamagnetic Fe3O4@ZnS core/shell nanocomposites were fabricated using biocompatible ethylenediaminetetraacetic acid (EDTA) as a capping agent by a facile refluxing assisted co-precipitation method at optimum condition. The Fe3O4 and Fe3O4@ZnS nanoparticles were investigated using X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), UV-Vis spectroscopy, vibrating sample magnetometer (VSM) and Dynamic Light Scattering (DLS). The VSM results indicated that all of the samples have superparamagnetic behavior. The particle size of the Fe3O4 and Fe3O4@ZnS nanoparticles were obtained at about 10 and 22 nm, respectively.