The upgrading of the physical and chemical properties of heavy oil by hydrotreating using a liquid Ni-Mo catalyst was studied through bench-scale tests aiming at developing "inside-reservoir" technology. The liquid Ni-Mo catalyst was synthesized by a very simple method consisting in preparing and mixing solutions at room temperature under acidic conditions. Mixtures of liquid Ni-Mo catalyst and heavy oil were evaluated in a 500-mL batch reactor at 350, 390 and 400 degrees C for 30, 60 and 90 min. The results showed that the liquid Ni-Mo catalyst improved the crude oil properties by increasing the API gravity from 12.5 to 22 and decreasing the kinematic viscosity from 13,490 to 72 cST at 15.6 degrees C; the sulfur content from 5.5 to 3.1% and the nitrogen content from 750 to 392 ppm were also reduced. Furthermore, the volume of gasoline and diesel by Simulated Distillation were increased to 8 and 14 vol. %, due to the change in chemical composition. Aromatic and saturated hydrocarbon compounds increased at the expense of asphaltenes and resins.