The complex triplet potential energy surface for the reaction of ground-state carbon atom C(P-3) with trans-C4H8 is theoretically investigated at the B3LYP/6-311G(d,p) and G3B3(single-point) levels. Various possible isomerization and dissociation pathways are probed. The initial association between C(P-3) and trans-C4H8 is found to be the C(P-3) addition to the C=C bond of trans-C4H8 to barrierlessly generate the three-membered cyclic isomer 1 CH3-cCHCCH-CH3. Subsequently, 1 undergoes a ring-opening process to form the chainlike isomer 3a cis-trans-CH3CHCCHCH3, which can either lead to P-6((CH3CHCCCH3)-C-2 + H-2) via the C-H bond cleavage or to P-7((CH3CHCCH)-C-2 + (CH3)-C-2) via C-C bond rupture. These two paths are the most favorable channels of the title reaction. Other channels leading to products P-1((CH3)-C-2-cCHCCH + (CH3)-C-2), P-2((CH3)-C-2-cCHCC-CH3 + H-2), P-3(trans-(CH3CHCH)-C-2 + (C2H3)-C-2), P-4(cis-(CH3CHCH)-C-2 + (C2H3)-C-2), P-5((CH3CH)-C-3 + (CH3CCH)-C-1), P-8(cis-(CH3CHCHCCH2)-C-2 + H-2), P-9(trans-(CH3CHCHCCH2)-C-2 + H-2), P-10((CH3CCCH2)-C-2 + (CH3)-C-2), and P-11((CH3CHCCHCH2)-C-2 + H-2), however, are much less competitive due to either kinetic or thermodynamic factors. Because the intermediates and transition states involved in the C(P-3) + trans-C4H8 reaction all lie below the reactant, the title reaction is expected to be rapid, as is consistent with the measured large rate constant. Our results may be helpful for future experimental investigation of the title reaction.