Provided are iron oxide nanocapsules for an MRI contrast agent having high contrast, in which a plurality of iron oxide nanoparticles having a hydrophobic ligand attached thereto are encapsulated in an encapsulation material including a biodegradable polymer and a surfactant, and which satisfy Relations 1, 2, 3, 4 and 5 below. Also a method of manufacturing the iron oxide nanocapsules is provided.
5≦100*D.sub.μ(IO)/C.sub.ω(IO)  [Relation 1]
2.5≦100*D.sub.μ(Cap)/C.sub.ω(Cap)  [Relation 2]
0.5 wt %≦F(IO)≦50 wt %  [Relation 3]
1 nm≦D.sub.μ(IO)≦25 nm  [Relation 4]
50 nm≦D.sub.μ(Cap)≦200 nm  [Relation 5]