Super Paramagnetic Iron-Oxide Nanoparticles (SPION) are currently used as magnetic resonance imaging (MRI) contrast agents. The functionalization of their surface with organic and biocompatible molecules has the purpose to produce carriers selective for different tissues and organs. In this paper, we present the preparation of new cystine-coated ultra small super paramagnetic iron-oxide nanoparticles (USPION) of different core size, from 4 nm to 11 nm. The physical-chemical characterization of these nanoparticles was performed by using several experimental techniques, such as atomic force microscopy (AFM), high resolution transmission electron microscopy (HRTEM) and magnetic measurements. 1H NMR relaxation times at different magnetic field strengths have been measured for several waterdispersions of cystine-coated iron-oxide nanoparticles of the smallest dimensions (4 nm). These preliminary results confirm their potentialities as molecular imaging probes and MRI contrast agents.