A process for upgrading hydrocarbon oil including mixing hydrocarbon oil with hydrogen gas and heating the hydrogen-enriched hydrocarbon oil before passing the oil through a cavitation apparatus to induce hydrotreating. Hydrotreating is achieved by hydrodynamically generating hydrogen-filled cavitation bubbles and collapsing the bubbles in the hydrocarbon oil under static pressure. The hydrotreating process can increase the API gravity of the hydrocarbon oil and reduce the viscosity of the hydrocarbon oil.