A novel aptamer modified thermosensitive liposome was designed as an efficient magnetic resonance imaging probe. In this paper, Gd-DTPA was encapsulated into an optimized thermosensitive liposome (TSL) formulation, followed by conjugation with AS1411 for specific targeting against tumor cells that overexpress nucleolin receptors. The resulting liposomes were extensively characterized in vitro as a contrast agent. As-prepared TSLs-AS1411 had a diameter about 136.1 nrn. No obvious cytotoxicity was observed from MTT assay, which illustrated that the liposomes exhibited excellent biocompatibility. Compared to the control incubation at 37 degrees C, the liposomes modified with AS1411 exhibited much higher T-1 relaxivity in MCF-7 cells incubated at 42 degrees C. These data indicate that the Gd-encapsulated TSLs-AS1411 may be a promising tool in early cancer diagnosis.