The potential energy surface (PES) for the reaction of the borohydride cation BH2+ and ethylene (C2H4) in the gasphase has been investigated at the B3LYP/6-311G(d. p) and single-point CCSD(T)/6-311G(2df. p) levels. Based on the calculated PES. the stabilities of various BC2H6+ isomers are determined and the mechanism of the formation of the association product BC2H6+ and the dissociation product BC2H4+ + H-2 in this reaction are also discussed. This study provides the first theoretical results on the reaction mechanism of the electron-deficient borohydride cations with alkenes.