Provided is an iron oxide nanocapsule which has extremely excellent water dispersibility, which is very stable in the body and which has very excellent MRI contrast ability, a method of manufacturing the same, and an MRI contrast agent using the same. The method of manufacturing the iron oxide nanocapsule includes the steps of: thermally decomposing an iron complex to prepare hydrophobic ligand-bonded iron oxide nanoparticles; and encapsulating the hydrophobic ligand-bonded iron oxide nanoparticles by a carboxymethyldextrandodecylamine conjugate encapsulation material or a dextran-linoleic acid conjugate encapsulation material to form an iron oxide nanocapsule.