Full geometry optimization of norbornadiene (NB) has been done by ab initio HF/3-21G*, HF/3-21G**, HF/6-31G*, HF/6-311G* and HF/6-311G* methods and the structure of the molecule is investigated. The double bonds of molecule are endo pyramidalized. The electron densities (q(i), (HOMO)) on exo and endo faces of the double bonds are not equal and it is bigger on the exo face. The NB-Cl-2 system is investigated by HF/3-21G* method and the stable configurations are determined. The stable configurations of the NB-Cl-2 system correspond to NB center dot center dot center dot Cl-2 (exo) and NB center dot center dot center dot Cl-2 (endo) molecular complexes that are formed by the exo and endo orientation Of Cl-2 molecule to the double bond of NB, respectively. The exomolecular complex has been found to be relatively more stable than the endo-complex. The cationic intermediates of the reaction have been studied by ab initio methods. The exo-chloronium cation is found to be more stable than endochloronium cation. The exo-facial selectivity should be observed in the addition reaction to NB of chlorine. The results obtained indicate that a multicenter nonclassical chlorocarbonium cation formed by the rearrangement of exo-chloronium cation is the most stable one among the cationic intermediates. It is likely that the ionic addition reaction proceeds via the nonclassical chlorocarbonium ion and results in the formation of the rearranged products. The mechanism of the addition reaction is also discussed.