In this study, the sub-cellular accumulation of superparamagnetic iron oxide nanoparticles (SPIONs) in breast tumors and peripheral organs were investigated. MNPs were conjugated with luteinizing hormone releasing hormone (LHRH), whose receptors are expressed by most types of breast cancer cells. After the nanoparticles were injected into female nude mice bearing MDA-MB-435S.luc tumors, the mice were sacrificed to collect tumors and peripheral organs for biological and TEM analyses. LHRH conjugated SPIONs (LHRH- SPIONs) were found to accumulate in cancer cells, mainly in the primary tumors and the metastatic lungs, where they aggregated to form clusters. In contrast, most of the unconjugated SPIONs were collected in the liver cells. The results suggest that LHRH- SPIONs can be used to target cancer cells in the primary breast tumors and the lung metastases. TEM is also shown to be a useful tool for the studies of sub-cellular distributions of SPIONs in tumors and tissues.