Gadolinium oxide (Gd2O3) nanoparticles (GNPs) are extremely useful for biomedical applications because they possess various excellent physical properties applicable to cancer theragnosis.1–5 These applications include positive (T1) magnetic resonance imaging (T1 MRI),6–13 computed tomography (CT) scan,14,15 and gadolinium neutron capture therapy (GdNCT) of cancers.16–29 For biomedical applications, coating of the GNP surfaces with hydrophilic and biocompatible ligands is essential. In this study, we used a cell-penetrating trans-activator of transcription (TAT) peptide as a surfacecoating ligand, thus allowing for greater accumulation of TATconjugated GNPs (i.e., TAT-GNPs) in cancer cells than in normal cells owing to cell penetration during circulation through angiogenesis.30 This role of the TAT peptide is consequently similar to cancer targeting. To prove this, enhanced cancer imaging of TAT-GNPs in T1 MRI was explored in this study. The peptide sequencing of the TAT peptide used was 48–57 fragments of the basic domain in the immunodeficiency virus of human (HIV)-1 TAT protein.30 So far, various kinds of TAT peptide-conjugated nanomaterials have been reported for use in drug delivery,31–33 gene delivery,34,35 and bio-imaging of cancer cells.36 Among them, anti-cancer drug delivery was very effective in destroying cancer cells owing to cancer-cell penetration during circulation through angiogenesis.31,32 However, TAT-GNPs have not been reported so far. Here, GNPs are considered potential T1 MRI contrast agents because their r1 values are higher than those of Gdchelates.6–13 Therefore, TAT-GNPs with cell-penetrating ability are expected to be extremely useful in diagnosing cancers through T1 MRI, as demonstrated in this study. This study reports one-pot synthesis and characterization of TAT-GNPs such as particle diameter, surface-coating structure, in vitro cellular toxicity, and water proton relaxivity. To demonstrate enhanced cancer imaging of the TATGNPs in T1 MRI, in vivo T1 MRI were recorded prior and posterior to intravenous administration into a model nude mouse tail with liver cancer.