Recently, oil sand in Canada has been gathering attention because of its huge storage, and the lack of fossil fuels. However, the bitumen from the oil sand has the problems for commercial use due to its high viscosity, high sulfur and high heavy metal content. Our research group has been carrying heavy oil/bitumen upgrading by hydrothermal visbreaking in supercritical water with the addition of alkali. Results showed that by supercritical water treatment with alkali was very effective for upgrading heavy oil/bitumen. However, water activation for upgrading bitumen remains poorly understood. An understanding of water activation is needed for improving upgrading bitumen in supercritical water reaction. In this study, therefore, the effect of water on upgrading bitumen in supercritical water reaction was investigated by varying reaction pressure and temperature. Experiments were performed in both a batch reactor and a continuous-flow reactor. In batch experiment, experiments were performed at temperature of 380°C to 460°C, the reaction pressure of 24 MPa to 40 MPa, and the reaction time of 30 min. Results showed that pressure and temperature, particularly pressure, have great effect on upgrading bitumen in supercritical water reaction This suggests that that the desired product could be obtained by controlling the reaction pressure and temperature.