Magnetic resonance imaging (MRI) using magnetic nanoparticles has been used to diagnose vascular diseases as well as to monitor transplanted cells and tissues. In this study, we synthesized magnetic iron oxide nanoparticles (TMADM-03), electrically charged by the presence of a cationic end-group substitution of dextran, and observed these nanoparticles inside three-dimensional models of HepG2 spheroids, which mimic tissues. Patterned cell array glass disks were prepared to visualize the presence of TMADM-03 uptaken by HepG2 spheroids using transmission electron microscopy (TEM). The HepG2 cells (2 × 10(5) cells) were inoculated onto Cell-able™ 12-well plates. After 48 h of culture, the cells were incubated with 75 µg Fe/ml TMADM-03 in culture medium for 24 h. To investigate the cellular function of the HepG2 spheroids, the albumin secretion was evaluated by an ELISA. The albumin secretion after incubation for 24 h was reduced compared with the secretion prior to the addition of TMADM-03. TEM image samples were prepared in a planar direction or a vertical direction to the HepG2 spheroids on patterned cell array glass disks. The incorporation of TMADM-03 inside the HepG2 spheroids was confirmed. In addition, TMADM-03 could be observed in the deeper layers of the spheroids, and this was localized in the lysosomes. These data suggest that the novel magnetic iron oxide nanoparticles invade three-dimensional HepG2 spheroids.