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*Dμ(IO)/Cω(IO)  [Relation 1] 2.5≦100*Dμ(Cap)/Cω(Cap)  [Relation 2] 0.5 wt %≦F(IO)≦50 wt %  [Relation 3] 1 nm≦Dμ(IO)≦25 nm  [Relation 4] 50 nm≦Dμ(Cap)≦200 nm  [Relation 5]