Magnetic iron oxide particles are widely used as contrastagents to improve the sensitivity of magnetic resonance imaging(MRI). Their efficiency in MRI is usually quantified by transverserelaxivity (r2) in solution. Herein, we synthesized a seriesof magnetite nanocrystal clusters (MNCs) with ultra-high transverserelaxivity by a polyol process and studied the relationshipbetween r2 and size of the MNCs. The sizes of MNCs canbe tuned over a wide range from 13 to 179 nm. The r2 of MNCsuspensions as a function of the size of the cluster was analyzedand compared with a theoretical model. We found thatMNCs of 64 nm had an r2 value of 650 mm 1 s 1, which wasmore than three times that of the commercial contrast agent and was among the highest reported for iron oxide materials.Compared with the theoretical model, the r2 value of the MNCsuspension is approximately 0.93 of the theoretical prediction.Imaging of the MNC suspensions was performed in a clinical1.5 T MRI instrument and a comparison was made betweenMNCs and commercial contrast agents. MRI indicated that thedecrease of signal intensity induced by MNCs was in proportionto the r2 value, which was in accordance with theoreticalpredictions. These results demonstrate that MNCs with ultrahightransverse relaxivity and tunable size are promising candidatesfor molecular imaging and clinical diagnosis in MRI.