Controlling ring-size selectivity is always a significant challenge for cyclooligomerization reaction. Herein, Ni-catalyzed cyclooligomerization of enones with methylene equivalents was investigated to elucidate the inner features of the transformation and to clarify the essence of ring-size selectivity by employing density functional theory (DFT) calculation. The computational results indicate that the two processes of reductive elimination and carbene insertion are competitive, and the four-membered metal ring intermediate prefers to undergo carbene insertion to form larger metallacycle rather than reductive elimination to afford by-production cyclopropane, thereby determined the ring-size selectivity. NPA charge analysis and correlation studies further support our conclusions. Meanwhile, we found that higher calculated barriers (27.3 and 29.1 kcal/mol, respectively) of reductive elimination suppress the formation of cyclobutanes and cyclohexanes, resulting in the failure to detect them in the experiment. Mechanistic investigations also reveal that ligands facilitate the release of nickel during the reductive elimination process, whereas the ligand-free reaction in other steps is more favorable. (C) 2019 Elsevier Inc. All rights reserved.