Iron oxide nanoparticles (NP) are of great interest in nanomedicine. They are used in Magnetic Resonance Imaging (MRI) as negative contrast agent, in tumor targeting, in drug delivery and in magnetic hyperthermia. In my research, we explore the effects of size distribution, magnetization and agglomeration of different-sized magnetite NP (Fe3O4) on NMR relaxation and magnetometry experiments. Indeed, these parameters play a key role in their efficiency as MRI contrast agents.