The sub- and supercritical water extractions of Athabasca oil sand bitumens were studied using a micro reactor. The experiments were carried out in the temperature range of 360-380 oC, pressure 15-30 MPa and water density 0.07-0.65 g/cm3 for 0-2 hrs. The extraction conversion of bitumens increased with solvent power and temperature. A maximum conversion of 24% was obtained after 90 min extraction at the supercritical condition. Hydrogen and carbon mono-oxide were not detected in sub-critical region but in the supercritical region. The supercritical condition was favorable to the hydrogen formation for bitumen extraction. The extraction products were upgraded relative to the original bitumens due to direct hydrolysis of low-energy linkage and H2 formed by water gas shift reaction in supercritical condition. 18% of initial sulfur in bitumen can be removed at maximum conversion condition. The asphaltene contents of the residue were significantly higher than that of original bitumen due to preferential extraction of aromatic compounds in supercritical condition.