The effect of the hydrogen donor additive on the viscosity of the heavy oils in the process of steam stimulation was studied. The results indicate that the heavy oil obtained from the Liaohe oilfields in China can undergo aquathermolysis at 240 degreesC for 24 h. After aquathermolysis, the viscosity of the oil sample decreased by 80%, and the SARA compositions changed. However, when there is no hydrogen donor additive in the reaction system, the viscosity of the reacted oil measured at different times after reaction will regress rapidly. With 0.8% tetralin, one of the most popular hydrogen donor additives, the viscosity did not regress again. The results are very useful for the popularization and application of the new technology for the in situ upgrading of heavy oil by aquathermolysis.