Provided are a preparation method of iron oxide-based paramagnetic or pseudo-paramagnetic nanoparticles, iron oxide-based nanoparticles prepared by the same, and a T1 contrast agent including the same. More particularly, the disclosure describes a method for preparation of iron oxide nanoparticles having a extremely small and uniform size of 4 nm or less based on thermal decomposition of iron oleate complex, iron oxide-based paramagnetic or pseudo-paramagnetic nanoparticles prepared by the same, and a T1 contrast agent including iron oxide-based paramagnetic or pseudo-paramagnetic nanoparticles.

Erythrocytes exposed to dialysis with an hypotonic buffer stably take up superparamagnetic iron oxide nanoparticles and may be used MRI contrast agents. Such erythrocytes may also be used as drug delivery vehicles.