Poly(á-amino acid)s and their derivatives have become increasingly popular as polymeric drug carriers because they have structures that can be potentially degraded by proteolytic enzymes in the body. The biocompatible and water dispersible poly(amino acid) derivative, PHEA-C18-C8COOH, was employed to alter the surface property of hydrophobic iron oxide nanoparticles. PHEA-C18-C8COOH coated iron oxide nanoparticles show high water-stability and exhibit high saturation magnetization and T2 relaxivility. The different size Fe3O4 nanoparticles (4-10 nm) protected by hydrophobic ligands were chemically conjugated onto the hydrophilic poly(amino acid)s through a ligand-exchange reaction. The hydrophilic poly(amino acid)s conjugated Fe3O4 nanoparticles form small spherical self-aggregates with a core-shell morphology in aqueous solutions. The self-assembled poly(amino acid)s conjugated Fe3O4 nanoparticles show good stability in water even after several lyophilizations and contain a high wt% of iron oxide nanoparticles in polymer. These self-aggregate would hold great potential for bioapplication such as magnetic resonance imaging