The decarbonylation of acetaldehyde assisted by Co+, which was selected as a representative system of the transition metal ions-assisted decarbonylations of acetaldehyde, has been investigated using density functional theory at the B3LYP/6-31+G** level of theory. The geometries and energies of the reactants, intermediates, products and transition states relevant to the reaction were located in detail on the triplet ground potential energy surfaces of [Co, O, C2, H4]+. Our calculations indicate the decarbonylation of acetaldehyde takes place through three steps, that is, C-C activation, aldehyde H-shift and nonreactive dissociation, while C-H activation by Co+ can't lead to the decarbonylation of acetaldehyde. The stable precursor of the decarbonylation products is determined to have a structure of (CH4) Co+(CO) rather than (CH3) HCo+(CO) proposed by earlier researchers [refs. 1,2].