A series of Mannich base-transition metal coordination complex was synthesized and used for the catalytic aquathermolysis of heavy oil, among which we found that the Ni(II) complex is the most effective one. After reacted under a relative low temperature, 180A degrees C, for 24 h, the viscosity of the heavy oil was decreased by 75.2% using 0.5 wt % Ni(II) complex. The results showed that the content of resin and asphaltenes was decreased, while the content of saturate and aromatic were increased after aquathermolysis reaction. Besides, the catalytic aquathermolysis could both decrease the viscosity of heavy oil, and reduce the content of heteroatoms, resulting in flow properties enhanced and the quality upgraded.