The role of supercritical water in upgrading bitumen is investigated by varying experimental parameters such as reaction pressure, which influences the density of water, and hydroxyl ions. Experiments were carried out using a batch reactor system operating with varying reaction pressure at a temperature of 703 K, and reaction time of 15 min. From the results such as the viscosity and the boiling point distribution of oil product etc., it can be seen that higher density supercritical water inhibits the cracking of bitumen to lower molecular weight substance. It has suggested that it is possible to obtain the product that has the desired quality in supercritical water on upgrading bitumen by adjusting reaction pressure. In the case of solvent with alkali it is found that alkali also helps to inhibit the cracking to lower molecular weight substances. Furthermore, the amount of the hydrogen became higher. It was inferred that some mechanism of hydrogen generation takes place with the addition of alkali. In experiments using a continuous flow reactor system at a temperature of 703 K, it was confirmed that higher density supercritical water also inhibits the pyrolysis.