Abstract: The various intermediates of electrcphilic addition of F⁺, Cl⁺ to ethylene, propene, fluoroethylene were optimized by MNDOmethod with energy gradient technique.The most stable configurations were obtained. The substituted ethylenes tended to form the open intermediates more easily than ethylene, but Cl⁺as a reagent, tended to form the cyclic intermediate more easily than F⁺.The interaction energies △Eof the above three compounds with F⁺ as electrophile were studied further with ab initio method and △Ewere decomposed into their components by Morokuma's energy decomposition scheme.In these reactions, it was found that charge transfer is the most important interaction, while electrostatic interaction is the next.The effects of the substituting groups are also discussed.