Nickel (Ni) nanoparticles (NPs) supported on different supports, including three carbon nanomaterials, i.e., ketjenblack (KB) carbon, carbon nanotubes (CNTs), and graphene nanoplatelets (GNPs), and zeolite, are prepared and studied as catalysts in the upgrading of heavy crude oil. X-ray diffraction and transmission electron microscopy measurements confirm the formation of dispersed Ni NPs with similar crystal size of approximately 12 nm in all the supported catalysts. In the upgrading tests, the reaction temperature and time as important reaction parameters are optimized to 300 degrees C and 2 h, respectively. The importance of an external hydrogen source is also verified as the upgrading effect is further enhanced in hydrogen than that in nitrogen. When pristine supports are employed as the catalysts, KB exhibits slightly better viscosity reduction compared with CNT, GNP, and zeolite. More importantly, in the case of supported catalysts, Ni/KB displays the highest viscosity reduction ratio of 75% relative to Ni/CNT, Ni/GNP, and Ni/zeolite catalysts, indicating a possible synergistic effect between the Ni NPs and the support. All the viscosity reduction results are in good agreement with the high-temperature simulated distillation analysis, demonstrating the effective upgrading of heavy crude oil. Furthermore, from the molecular structural information obtained from Fourier transform ion cyclotron resonance mass spectrometry, the viscosity reduction and catalytic upgrading are attributed to the conversion of large molecule carboxylic acid compounds to derivatives with smaller carbon numbers and higher saturation. This result is further confirmed by the significant oxygen reduction by the elemental analysis. Therefore, it is concluded that the decomposition of carboxylic acid compounds contributes greatly to the viscosity reduction and Ni/KB can catalyze the decomposition process as an effective catalyst. This work highlights the potential application of carbon-based nanocatalysts for the in situ upgrading and recovery of heavy crude oil, especially crude oils with high oxygen content.