Conventional crude oil production is declining, while the consumption of petroleum-based fuels is increasing. Therefore, bitumen and heavy oil exploitation is steadily growing. However, in the present context, heavy oil and bitumen exploitation processes are high-energy and water-intensive and, consequently, have significant environmental footprints because of the production of gaseous emissions, such as CO2, and generating huge amounts of produced water. In situ catalytic conversion or upgrading is a promising cost-effective and environmentally friendly technology that aims at reducing the environmental footprints of oil sand exploitation and producing of high-quality oil that meets pipeline and refinery specifications. In this study, in situ prepared Ni-W-Mo ultradisperse nanocatalysts within a vacuum gas oil matrix were used for Athabasca bitumen upgrading in a packed-bed flow reactor at a high pressure and temperature. Experiments were performed at a pressure of 3.5 MPa, temperatures from 320 to 340 degrees C, and a hydrogen flow rate of 1 cm(3)/min. The produced liquid was analyzed on the basis of residue conversion, microcarbon residue (MCR) content, sulfur and nitrogen contents, American Petroleum Institute (API) gravity, and viscosity. Results showed that nanocatalysts enhanced the quality of Athabasca bitumen by increasing the API gravity and decreasing the viscosity and MCR, sulfur, and nitrogen contents. Nanocatalysts effectively favored the hydrogenation reactions and inhibited the massive formation of coke that usually occurs via olefin polymerization and heavy free radical condensation during the classical thermal cracking process of heavy oils.