Direct ab initio trajectory calculations have been applied to a S(N)2 reaction, OH- + CH3Cl --> CH3OH + Cl-. First, static ab initio molecular orbital (MO) calculations with several basis sets were examined to select the most convenient and best fit basis set to that of high-quality calculations. As a result of the static ab initio calculations, it was found that the Hartree-Fock (HF)/3-21+G(d) calculation reasonably represents a potential energy surface calculated at the MP2/6-311++G(2df,2pd) level. Next, direct ab initio dynamics calculations using the 3-21+G(d) basis set were carried out for the S(N)2 reaction. A full dimensional potential energy surface including all degrees of freedom was used in the dynamics calculation. The collision energies chosen were E-coll = 5 and 25 kcal/mol. In the collisions at E-coll = 5 and 25 kcal/mol, 48% and 63% of the total available energies were, respectively, partitioned into the relative translational mode between CH3OH and Cl-. Also, it was predicted that the C-H stretching mode of the product CH3OH is excited after the S(N)2 reaction, which is not detected in the case of the halogen-atom exchange S(N)2 reaction. The reaction mechanism was discussed on the basis of theoretical results.