Superparamagnetic iron oxide(SPIO) nanoparticle clusters are one unique form which can enhance magnetic relaxivity and improve the magnetic resonance imaging contrast at the same iron concentration, comparing to single SPIO nanoparticles. Controlling of cluster size and other structural parameters have drawn great interests in this field to further improve their magnetic properties. In this study, we investigated how the interparticle distance(also known as neighbor distance) of SPIO nanocrystals within clusters affect their magnetic relaxation behaviors.To adjust the neighbor distance, different amount of cholesterol(CHO) was chosen as model spacers embedded into SPIO nanocluster systems with the help of amphiphilic diblock copolymer poly(ethylene glyco)-polyester. Smallangle X-ray scattering was applied to quantify the neighbor distance of SPIO clusters. The results demonstrated that the averaged SPIO nanocrystal neighbor distance of nanoclusters increased with higher amount of added CHO.Moreover, these SPIO nanocrystal clusters had the prominent magnetic relaxation properties. Simultaneously, controlling of SPIO nanocrystal neighbor distance can regulate the saturation magnetization(Ms) and magnetic resonance(MR) T2 relaxation of the aggregation, and ultimately obtain better MR contrast effects with decreased neighbor distance. Superparamagnetic iron oxide (SPIO) nanoparticle clusters are one unique form which can enhance magnetic relaxivity and improve the magnetic resonance imaging contrast at the same iron concentration, comparing to single SPIO nanoparticles. Controlling of cluster size and other structural parameters has drawn great interests in this In this study, we investigated how the interparticle distance (also known as neighbor distance) of SPIO nanocrystals within clusters affect their magnetic relaxation behaviors. To adjust the neighbor distance, different amount of cholesterol (CHO) was chosen as model spacers embedded into SPIO nanocluster systems with the help of amphiphilic diblock copolymer poly (ethylene glyco) -polyester. Smallangle X-ray scattering was applied to quantify the neighbor distance of SPIO clusters. The results demonstrated that the averaged SPIO nanocrystal neighbor distance of nanoclusters increased with higher amount of added CHO.Moreover, these SPIO nanocrystal clusters had the prominent magnetic relaxation properties. Simultaneously, controlling the SPIO nanocrystal neighbor distance can regulate the saturation magnetization (Ms) and magnetic resonance (MR) T2 relaxation of the aggregation, and ultimately obtain better MR contrast effects with decreased neighbor distance.