Co(II) coordination complex was synthesized, characterized, and then used in aquathermolysis of heavy oil as catalyst at relatively low temperature, 180A degrees C. The effects of water amount, catalyst concentration, ethanol amount on aquathermolysis were investigated in this work. The crude oil before and after aquathermolysis was fully characterized, and the mechanism of viscosity reduction was discussed at last. The results show that heavy oil can undergo aquathermolysis in the presence of water and the Co(II) complex at low temperature. Furthermore, it was found that ethanol can enhance the catalytic aquathermolysis. Besides, the catalytic aquathermolysis could not only decrease the viscosity of heavy oil, but also remove some heteroatoms, finally make the flow properties better and the quality upgraded. The experimental results prove that the combination of catalyst and ethanol has a synergetic effect, which contributes to the great reduction of viscosity and improvement of heavy oil quality.