The addition reactions of NO3 to limonene have been investigated using ab initio methods. Six different possibilities for NO3 addition to the double bonds, which correspond to the two C-C double bonds (endocyclic or exocyclic) have been considered. The negative activation energies for the addition of NO3 to limonene are calculated and the energies of NO3-limonene radical adducts are found to be 14.55 to 20.17 kcal mol(-1) more stable than the separated NO3 and limonene at the CCSD(T)/6-31G(d) + CF level. The results also indicate that the endocyclic addition reaction is more energetically favorable than the exocyclic one.