CO₂ reduction with green H₂ is an attractive way to produce fuels or chemicals. Owing to the complexity of the reaction network of CO₂ hydrogenation, various products could be generated. Understanding the origin of CO₂ hydrogenation selectivity is critical to rationally design catalysts with high activity and desired selectivity. Here, through a systematic investigation of the size-dependent catalytic performance of Ni-based catalysts, the primary pathway for Ni-catalyzed CO₂ hydrogenation, i.e., the reverse water-gas shift (RWGS) followed by H₂-assisted CO dissociation and hydrogenation pathway, is experimentally determined. The rate-determining step in the CO₂ methanation reaction is identified to be H₂-assisted CO dissociation. Therefore, the activation barrier of H₂-assisted CO dissociation can be employed as a quantitative descriptor of CH₄ selectivity. The identification of an experimental accessible quantitative descriptor sheds light on the rational design of high-efficient heterogeneous catalysts with fine-tuned product selectivity.