We describe the preparation of superparamagnetic nanoclusters (SNCs) by fine-tuning of the seed Fe3O4 nanoparticle sizes to enhance and their T2 relaxivity can be increased by > 4-fold. Therefore, with 11 nm seed core and PVA coating, SNC-11 exhibit a higher T2 relaxivity than other cluster condition. So fabricating the cluster, seed size is the most important influence the T2 relaxivity. As well as, in vitro cellular imaging results demonstrated the strong potential of SNCs for clinical applications by targeting affinity. According to the experiments, with 11 nm seed core and PVA coating, SNC-11 exhibited the highest T2 relaxivity of 454 mM-1s-1 due to the strong seed size effect on their magnetic sensitivity, indicating superior magnetic resonance (MR) contrast efficiency. Further in vitro cellular imaging results demonstrated the strong potential of SNCs for clinical applications.