© 2014 WILEY-VCH Verlag GmbH & Co. KGaA , Weinheim. In this paper, a highly innovative concept of using ultrathin iron oxide nanowhiskers as a positive (T 1 ) contrast agent for magnetic resonance imaging (MRI) is demonstrated. Iron oxide nanowhiskers with dimensions of approximately 2 nm × 20 nm are synthesized by heating an iron oleate/oleylamine complex under 150 ° C. These nanostructures have very high surface-tovolume ratios, leading to strong paramagnetic signal, a property suitable for T 1 contrast in MRI. The positive contrast enhancement of these nanowhiskers is demonstrated in vitro and in vivo in a rat model. Successful development of this technology has substantial commercial value in biomedical imaging, potentially leading to the advancement of human healthcare technologies.