Gadolinium(III) complexes, such as Gd-DTPA (Magnevist (R)) and Gd-DOTA (Dotarem (R)), are widely used as magnetic resonance imaging (MRI) contrast agents (CAs) in clinical practice. Both high relaxivity and stability are crucial requirements for Gd(III)-based materials to be used as MRI-CAs. The relaxivity is generally improved by increasing the hydration number (q) and/or embedding Gd(III) complexes in/onto slow rotating objects, while the coordination stability can be enhanced when nitrogen donors are replaced with oxygen donors according to the hard-soft-acid-base (HSAB) principle. In this work, we synthesized a tripodal hexadentate all-oxygen-donor ligand bearing three pairs of carboxylate and amine-N-oxide chelating groups and used microemulsion technique to further form nanosized coordination networks. The obtained Gd(III)-containing nanoparticles exhibited a narrow size distribution centered at 90 nm according to dynamic laser scattering and transmission electron microscopy analysis. The relaxivity of the nanoparticles reached 25.95 s(-1)per mM of Gd(III), about six times higher than that of the commercial MRI-CAs. The competing coordination experiments showed that Gd(III) ions were firmly bonded in the coordination networks, complying well with the safety requirement for clinical applications. Moreover, the nanoparticles had low cytotoxicity and displayed good biocompatibility for potential in vivo applications.