To estimate the kinetic isotope effect (KIE) for nucleophilic substitution (S N 2) reaction, F − +CH 4 →CH 3 F+H − , and deuterated ones, we have considered the geometrical isotope effect (GIE) induced by the difference of the protonic and deuteronic wavefunctions using the multi-component molecular orbital (MC_MO) method. By replacing protons with deuterons, the C–D bond lengths are about 0.007Å shorter than C–H. We estimated the ratio (kaH/kaD) of rate constants based on MC_MO results. The geometrical and energetic differences between H and D compounds are induced by relaxation of electronic structures due to the difference in the wavefunctions of the proton and deuteron.