Abstract: A series of Gd(Ⅲ) complexes were synthesized and evaluated as potential magnetic resonance imaging (MRI) contrast agents.The ligands of these complexes are featured with two hyphenated amine-N-oxide and four carboxyl groups,providing the complexes with high hydration numbers (q,the number of inner-sphere water molecules).The structures of the ligands and complexes were characterized with ¹H-NMR,¹³C-NMR and ESI-MS.Preliminary results on 1.5T MRI measurement of the complexes showed high r₁ values (longitudinal relaxivity) of 8.5-10.1 (mM)^-1·s^-1.The q value of the complexes was determined as up to 3 with fluorescence lifetime analysis with their Eu(Ⅲ) analogues.Cell tests revealed low cytotoxicity of the complexes,which is essential for clinical applications.The structural simplicity,the high relaxivity and the low toxicity of the complexes provide a new solution for the development of highly efficient MRI contrast agents.