The high viscosity of heavy oil is the main challenge hindering its production. Catalytic thermolysis can be an effective solution for the upgrading of heavy oil in reservoir conditions that leads to the viscosity reduction of native oil and increases the yield of light fractions. In this study, the thermolysis of heavy oil produced from Ashalchinskoye field was carried out in the presence of FeP and Al(Hsub.2POsub.4) nanocatalysts at a temperature of 250 °C in Nsub.2 gas environment. It was shown that Al(Hsub.2POsub.4)sub.3 and FeP catalysts at a concentration of 0.5% significantly promoted the efficiency of the heavy oil thermolysis and are key controlling factors contributing to the acceleration of chemical reactions. The Al(Hsub.2POsub.4)sub.3 + NiCOsub.3 nanoparticles were active in accelerating the main chemical reactions during upgrading of heavy oil: desulfurization, removal of the side alkyl chains from polyaromatic hydrocarbons, the isomerization of the molecular chain, hydrogenation and ring opening, which led to the viscosity reduction in heavy oil by 42%wt. Moreover, the selectivity of the Al(Hsub.2POsub.4)sub.3 + NiCOsub.3 catalyst relative to the light distillates increased up to 33.56%wt., which is more than two times in contrast to the light distillates of initial crude oil. The content of resins and asphaltenes in the presence of the given catalytic complex was reduced from 34.4%wt. to 14.7%wt. However, FeP + NiCOsub.3 nanoparticles contributed to the stabilization of gasoline fractions obtained after upgraded oil distillation. Based on the results, it is possible to conclude that the thermolysis of heavy oil in the presence of FeP and Al(Hsub.2POsub.4)sub.3 is a promising method for upgrading heavy oil and reducing its viscosity.