The mechanism of selective co-oligomerization of ethylene and 1-hexene by the catalyst [CrCl3(PNPOMe)] (a, PNPOMe = N,N-bis­(bis­(o-methoxyphenyl)­phosphine)­methylamine) has been explored in detail using the density functional theory (DFT) method. The full catalytic cycles for the formation of 1-hexene and 1-decenes were calculated on the basis of the metallacyclic mechanism, and the distribution of all decene isomers was explained by locating Gibbs free energy surfaces of various pathways, which is in good agreement with the experimental results. A spin surface crossing through a minimum energy crossing point (MECP) from a sextet to a quartet surface takes place before the formation of metallacyclopentane, which opens up a much lower energy pathway and thus facilitates the following co-oligomerization reactions. It is worth noting that β-hydrogen agostic-assisted hydrogen transfer is of crucial importance for the decomposition of the metallacycle intermediates to give 1-hexene or decenes. Moreover, an analysis of the Cr–O bond distance and NBO charges indicates the important role of a hemiliable methoxy moiety, which acts as a pendant group in the co-oligomerization of ethylene and 1-hexene by CrCl3(PNPOMe) catalyst.