Vacuum residuum ( VR) upgrading through pyrolysis in supercritical water was investigated in a bomb reactor and a batch autoclave. The experiments were carried out in the temperature range of 380- 460 C at 25.0 MPa for 5- 120 min with a water/ VR ratio of 0.78: 1- 4: 1 by weight. The upgrading leads to a reduction of 30.9% in resins and asphaltenes, a reduction of 22.8% in aromatics, and an increase of 98.6% in saturates. The viscosity of the product was reduced from 116 mPa s of the feedback to 6.2 mPa s, which is approximately 179 times. Besides, the average molecular weight was reduced from 1860 to 646 g mol(-1), and S, N, Ni, and V contents were reduced by 32, 15, 83, and 85%, respectively. The upgrading mechanism was proposed as ionic and radical reactions in parallel.