The reaction mechanism and origins of ligand-controlled selectivity, regioselectivity, and stereoselectivity of Ni-catalyzed (2 + 2 + 2) cycloadditions and alkenylative cyclizations of 1,6-ene-allenes and alkenes were studied by using density functional theory. The catalytic cycle involves intermolecular oxidative coupling and an intramolecular concerted 1,4-addition step to afford a stable metallacycloheptane intermediate; these steps determine both the regioselectivity and stereoselectivity. Subsequent C-C reductive elimination leads to the cyclohexane product, whereas the β-hydride elimination leads to the trans-diene product. The selectivity between (2 + 2 + 2) cycloadditions and alkenylative cyclizations is controlled by the ligand. Irrespective of the nature of the terminal substituents on the ene-allene and alkene, the P(o-tol)3 ligand always favors the C-C reductive elimination, resulting in the cyclohexane product. On the other hand, the flexibility of the PBu3 ligand means that electronic and steric factors play important roles. Electron-withdrawing groups such as CO2Me in the ene-allene terminal substituent induce obvious substrate-ligand repulsion and destabilize the C-C reductive elimination, giving rise to the trans-diene product.