A new magnetic nanoparticle was synthesized in the form of Gd(3+)-chelated Fe(3)O(4)@SiO(2). The Fe(3)O(4) nanoparticle was octahedron-structured, was highly magnetic (∼94 emu/g), and was the core of an encapsulating mesoporous silica shell. DOTA-NHS molecules were anchored to the interior channels of the porous silica to chelate Gd(3+) ions. Because there were Gd(3+) ions within the silica shell, the transverse relaxivity increased 7-fold from 97 s(-1) mM(-1) of Fe(3)O(4) to 681 s(-1) mM(-1) of Gd(3+)-chelated Fe(3)O(4)@SiO(2) nanoparticles with r(2)/r(1) = 486. The large transversal relaxivity of the Gd(3+)-chelated Fe(3)O(4)@SiO(2) nanoparticles had an effective magnetic resonance imaging effect and clearly imaged lymph nodes. Physiological studies of liver, spleen, kidney, and lung tissue in mice infused with these new nanoparticles showed no damage and no cytotoxicity in Kupffer cells, which indicated that Gd(3+)-chelated Fe(3)O(4)@SiO(2) nanoparticles are biocompatible.