NiO nanoparticles have been synthesized as a catalyst in aquathermolysis for viscosity reduction of heavy oil. NiO nanoparticles were prepared by using NiCl2•6H2O as raw material via ethylene glycol route. In the process, NH4HCO3and ethylene glycol were used as reductant and to prevent nanoparticles from agglomeration, respectively. The obtained NiO nanoparticles have crystallite size of about 15.4 nm, nearly uniform particles size, spherical morphology, good dispersion with the diameter of around 65 nm, and the specific surface area of 158.4 m2/gram. In catalytic activity test, it was shown that the viscosity of heavy oil can be decreased with the reduction of 22% when NiO nanoparticles are added in the aquathermolysis reaction. This result confirmed that NiO nanoparticles have good catalytic properties in the viscosity reduction of heavy oil.