Novel magnetic nanocomposites containing Fe3O4 nanoparticles loaded on a hyper-branched polymer grafted on graphene-oxide are synthesized via polycondensation of citric-acid in presence of the graphene-oxide. Hydroxyl functional groups of graphene-oxide act as monomers to polymerize the citric-acid and result in a nanocomposite formed by graphene-oxide grafted with poly-citric-acid. The degree of polymerization is controlled by using different ratios of citric-acid to graphene oxide. Grafting poly-citric-acid, a biocompatible and water soluble polymer, has important applications. In addition, the branches of polymer, which include the acid functional groups, enable to capture and encapsulate a large number of nanoparticles. We used graphene-oxide grafted with poly-citric acid for synthesizing and stabilizing Fe3O4 nanoparticles in the size range of 16.0-18.5 nm obtained via X-ray diffraction pattern. Magnetic characterization as well as scanning and tunneling electron microscopy images confirm the loading of significant amounts of magnetic nanoparticles on the surface of graphene oxide grafted by hyperbranched poly-citric acid polymer. Moreover, a significant saturation magnetization of similar to 78 emu/g is observed in comparison with Fe3O4 doped raw graphene-oxide, synthesized by almost similar rout.