Nano-keggin-K3PMo12O40 Was synthesized, characterized by FT-IR, XRD and TEM, and then used in catalytic aquathermolysis of extra-heavy oil for the first time. The results evidently showed a viscosity reduction by 92.3% for the extra-heavy oil Zhen 411 at 280 degrees C, and by 80% and 90% in viscosity reduction for the extra-heavy oil G540 at 200 degrees C and 280 degrees C, with 9.25% and 11.69% in conversions of heavy to light content, respectively. To evaluate the performance, the structure and group composition of the heavy oil before and after the reaction were tested and analyzed by TLC-FID, FT-IR, GC-FID, H-1 NMR, EL and GC-MS. It was found that changes in oxygen-containing groups mainly happen during the catalytic aquathermolysis. (c) 2009 Elsevier Ltd. All rights reserved.