NixFe3-xO4nanoparticles have been synthesized using microwave-assisted coprecipitation. The samples were prepared using FeCl3·6H2O, FeSO4·7H2O, NiCl2·6H2O, and NH4OH as precursors. The concentrated black precipitates were produced after the samples were heated in microwave 500 W 2.45 GHz for 3 minutes. According to X-Ray diffractogram, it was indicated that the NixFe3-xO4was obtained. According to Scanning Electron Microscope (SEM), it was shown that Ni0.3Fe2.7O4nanoparticles have spherical morphology and average particle size about 43 nm. In this research, NixFe3-xO4nanoparticles were applied as catalyst in the viscosity reduction of heavy oil via aquathermolysis. In catalytic activity test, Fe3O4and Ni0.3Fe2.7O4nanoparticles were added respectively to the mixture of water and heavy oil in autoclave on temperature 120°C for 6 hours. Experimental result showed that the viscosity of heavy oil reduced 30% and 17% when Fe3O4and Ni0.3Fe2.7O4powders were added in aquathermolysis reaction, respectively.