Electronic structure calculations for the ortho, meta, and para isomers of nitrenophenylmethylene (C6H4-CHN) are carried out by using the full-optimized-reaction-space (FORS) multiconfiguration self-consistent-field (MCSCF) method with the STO-3G basis set. The ortho isomer is predicted to possess a singlet ground state with a bond-alternated structure. However, since the energy barrier for the isomerization reaction of the ortho structure to a bicyclic one (benzoazacyclobutene or benz[b]azete) is less than 10 kcal/mol, the ortho isomer is considered to have a relatively short lifetime and hence to exist in the stable bicyclic form. The para isomer also has a singlet ground state with a bond-alternated structure, but it may easily react with molecular oxygen, as suggested experimentally, because the lowest tripler state is very close in energy to the singlet ground state. In marked contrast, the meta isomer has a quintet ground state, and this aspect is well reflected in the bonding, which shows no appreciable pi conjugation between a benzene nucleus and two radical substituents.