Ab initio MO calculations have been carried out for the N(D-2) + HCN((1)Sigma) reaction system. The CASSCF(9,9)/cc-pVDZ level of theory was employed for initial processes, and addition of N(D-2) to the N atom was found to be energetically the most favorable pathway. Several intermediates and transition states located on overall reaction pathways except for the initial processes were optimized at the MP2(full)/cc-pVTZ level and their single-point energies were calculated at the PMP4(SDTQ,full)/cc-pVTZ level. It was found that the main product channel is CH((II)-I-2) + N-2((1)Sigma(g)), which is 53.1 kcal mol(-1) lower in energy than the N(D-2) + HCN((1)Sigma) reactants.