Geometrical and electronic structures of SF6 and SF6- have been determined using ab initio molecular orbital calculations in order to shed light on the mechanism of electron capture of SF6. The calculations (MP2/6-311G(d) + sp and MP2/6-311 + G(d) levels) showed that both SF6 and SF6- have Oh symmetry, while the S-F bond of SF6- is about 10% longer than that of SF6, indicating that the structure of SF6 is deformed along the S-F symmetric stretching mode by accepting an excess electron. The calculations showed that the adiabatic electron affinities of SF6 are positive (0.49-1.39 eV), which is in reasonable agreement with the experimental values (0.32-1.49 eV). The potential energy curve calculated as a function of an S-F bond of SF6- showed that SF6- is bound with respect to its dissociation limit (SF5- + F). The mechanism of electron capture of SF6 is discussed on the basis of the theoretical results.