We report a direct synthesis of ultrasmall c(RGDyK) peptide-coated Fe3O4 NPS (< 10 nm in hydrodynamic diameter) and demonstrate their in vivo tumor-specific targeting capability. The Fe3O4 NPs are synthesized by thermal decomposition fo iron pentacarbonyl in the presence of 4-methylcatechol (4-MC), and the peptide is coupled to nanoparticles through 4-MC via Mannich reaction. The c(RGDyK)-MC-Fe3O4 NPs have an overall diameter of similar to 8.4 nm and are stable in physiological conditions. When administered intravenously, these c(RGDyK)-MC-Fe3O4 NPs accumulate preferentially in the integrin alpha(v)beta(3)-rich tumor area, which are readily tracked by MRI.