Ab imritio MO calculation has been performed in an effort to determine the possible reaction path of the addition reaction of HF on C₂H₄.Two limiting reaction mechanisms have been considered.One is a molecular addition which proceeds stereo-specifically via a cyclic four-center transition state and exclusively occurs with cis stereoohemistry.The other is a molecular addition which proceeds via a transition state in which the HF molecule is normal to the plane of ethylene and above the C=C bond midpoint.From the results of calculation it is found that the firat mechanism is the possible reaction path.The calculated activation energy is 58.68 kcal/mol (246.32 kJ/mol), which is to be compared to the experimental value 54±3 kcal/mol.The charge transfer between HF and C₂H₄ in the complex has been discussed in terms of Mulliken's population.It was found that negative charge (electrons) is transferred from C₃H₄ to HF when the intermolecular distance R is much greater or much smaller than R₀ (the intermolecular distance in the transition state).But when R is near R₀, we found that tho direction of charge transfer is reversed.This phenomenon is explained from a frontier orbital point of view.Furthermore, it was shown that Woodward-Hoffmann rules do not fit to the present case because the fluorine atom brings more than one orbitals into play during the reaction process.