Full geometric optimization of exo-tricyclo[220.127.116.11(2,4)]oct-6-ene(exoTCO) has been done by the semiempirical methods and the structure of the molecule investigated. The double bond of molecule is endo-pyramidalized and the two faces of double bond are no longer equivalent. Exo face of the double bond of the molecule has regions having high electron density(q(i,HOMO)) and bigger negative potential. The exoTCO... Br-2 system has been investigated by AM1 method and exoTCO... Br-2(exo) molecular complex has been found to be relatively more stable than the exo...Br-2(endo) complex. The cationic intermediates of the reaction have been studied by semiempirical methods. Exo-bromonium cation is found to be more stable than endo bromonium cation. Exo-facial selectivity has been observed in the addition reaction to exoTCO of bromine which is caused by electronic and steric effects. Exo-classical bromocarbonium cation(III) is more stable than rearrangament cation(V) which is formed with Wagner-Meerwein rearrangament. Bromocarbonium cation(III) is the most stable ion among the cationic intermediates and the ionic addition occurs via the formation of this cation. The mechanism of the addition reaction has also been discussed.