Abstract: OBJECTIVE To study pharmacokinetics features and tissue distribution of OCMCS-USPIO-NPs in SD rats in vivo to provide evidence for their clinical use in future. METHODS SD rats were divided into three groups: blank group, OCMCMS-USPIO-NPs group and dextran-SPIO-NPs group, then iron content in plasma and different tissue including heart, liver, spleen, lung and kidney were determined by atomic absorption spectroscopy. The iron concentration-time in plasma and tissues was drawn. The plasma concentration-time data of iron were analyzed by DAS 2.1.1 statistical software and the main pharmacokinetics parameters was caculated. Statistics analysis combined with Prussian blue staining were used to demonstrate OCMCS-USPIO-NPs and dextran-SPIO-NPs tissue distribution difference in rats. RESULTS There was significant difference between OCMCS-USPIO-NPs group and dextran-SPIO-NPs group in the main pharmacokinetics parameters of iron, including AUC, MRT, t_1/2, CL, V₂(P<0.05), the t_1/2 in OCMCS-USPIO-NPs group were longer than 7 h in high or low dose group. Compared with dextran-SPIO-NPs group, not only statistical analysis but also Prussian blue staining results indicated the iron content in liver, spleen and lung in OCMCS-USPIO-NPs group were significant lower than dextran-SPIO-NPs. CONCLUSION OCMCS-USPIO-NPs can escape the RES capture to gain longer-circulation time.