A simple and convenient method is explored for preparing high relaxivity magnetic resonance imaging (MRI) contrast agents with numerous N-oxide groups decorated on the hyperbranched precursors as ligands for the first time. Such Gd(III) containing complexes use modified hyperbranched (or linear) polyethylenimine (PEI) with polydentate amine-N-oxide carboxylate groups as the ligands and complexes with different molecular weights show sizes from 1.63 nm to 5.40 nm with the increase of the molecular weight. T₁ magnetic resonance imaging experiments show that the hyperbranched MRI contrast agents have much higher relaxivity than the linear analogue. Meanwhile, with the increase of molecular weight, the relaxivity of the complexes slowly increases to 8.9 mM⁻¹ s⁻¹, a number much higher than that of commercial MRI contrast agent, Gd(III) diethylenetriamine pentaacetate (Gd-DTPA). The in vitro cytotoxicity test demonstrates the excellent biocompatibility of these complexes, which is essential for clinical applications.