The reaction of isocyanato radical, NCO (X 2 ∏) with chlorine atom, Cl ( 2 P u ) is investigated by ab initio quantum chemistry methods. The important triplet potential energy surfaces (PES) are calculated at the CCSD(T)/aug-cc-pVQZ//CCSD/6-311++G ∗∗ +ZPE levels. On the triplet PES of the title reaction, the most possible pathway is via 3 TSR-1 forming the cis isomer 3 1, and then 3 1 can lead to the lower trans isomer 3 2 via transition state 3 TS1-2, followed by the transition state 3 TS2-P1 to get the important product P1 with larger exothermic. The other reaction pathways are less competitive due to thermodynamical or kinetic factors. The possible singlet–triplet intersystem crossing is also discussed. We expect this contribution can lead us to deeply understand the mechanism of the title reaction and may be helpful for the modeling of isocyanato radical-halogen combustion chemistry.