Objective: To investigate the magnetic resonance (MR) signal changes of superparamagnetic iron oxide (SPIO) and its biological effects on endothelial cells. Methods: The citric-acid coated SPIO was synthesized by co-precipitation method.The human umbilical vein endothelial cells (HUVECs) were incubated with SPIO for 24 h in culture medium at iron concentration of 0.01,0.05,0.10,0.15 mg/ml (experimental groups),and the cells incubated without SPIO served as control groups.The uptake efficiency of intracellular iron was measured by Prussian blue staining,and the cell viability was monitored by Calcein-AM method.The cell cytoskeleton (F-actin and tubulin),adherence and migration capacity were measured by immunofluorescence staining.The iron oxide nanoparticles distribution and the cellular organelle change were monitored by transmission electron microscopy (TEM).Quantification of particle uptake was measured by atomic absorption spectrometry.The MR signal of endothelial cells after labeling was monitored by Philips 3.0 T MR scanner. Results: SPIO was uptaken by HUVECs in a concentration-dependence manner.Compared with the control group,cell viability was decreased along with the increase of iron concentration.Compared with the control group,the cell cytoskeleton was markedly disorganized and the FAK spot was bigger and sparser.The nanoparticles were mainly existed in lysosomes,and the higher concentration of SPIO,the more lysosomes and vacuoles presented in the cells.The iron content per cell was (55.86±9.935)pg when the SPIO concentration was 0.15 mg/ml.The MR image showed that the cells labeled with SPIO resulted in the decrease of MR signal. Conclusion: The cells labeled with SPIO can be detected by MR.The cell viability,cytoskeleton,adherence and migration capacity of HUVECs are affected by citric-acid coated SPIO in a concentration-dependent manner.