The mechanism and stereochemistry of electrophilic addition of chlorine to bisbenzotetracyclo[22.214.171.124(3.6).0(2.7)]tetradeca-4,9,11,13-tetraene (BBTT) molecule were investigated by DFT methods. The geometry and the electronic structure of BBTT molecule was studied by DFT/B3LYP method using the 6-311G(d) and 6-311++G(d,p) basis sets. The double bonds of BBTT molecule are endo-pyramidalized. The structure and stability of the cationic intermediates and products of the addition reaction were investigated by B3LYP/6-311G(d) and B3LYP/6-311+G(2d,p) methods. The solvent effect was evaluated using SCI-PCM method. The bridged chloronium cation is isomerized into the more stable nonclassical delocalized N- and U-type cations, and the difference between the stability of these cations is small. For the determination of the direction of addition reaction and the stereochemistry of the products, the stability of nonclassical delocalized N- and U-type ions and the structure of their cationic centres play a vital role for the determination of the direction of addition reaction and the stereochemistry of the products. Since the cationic centre of the N-type ion is in interaction with the benzene ring from the exo face, the nucleophilic attack of the chloride anion to this centre occurs from the endo face, and the exo,endo-isomer of the N-type product is obtained. The attack of chloride anion towards the cationic centre of U-type ion from the endo face is sterically hindered by the hydrogen atom, therefore the attack occurs from the exo face, which interacts with the benzene ring and the more stable exo,exo-isomer of U-type product is formed. Although, the U-type cation was 3.485 kcal mol(-1) more stable than the N-type cation, the U-type product was 1.886 kcal mol(-1) [SCI-PCM-B3LYP/6-311++G(2d,p)// B3LYP/6-311G(d)] less stable than the N-type product.