A microsolvated S(N)2 reaction of F-(H2O) with CH3Cl has been investigated by means of direct ab initio dynamics calculations in order to elucidate a detailed reaction mechanism. A full dimensional ab initio potential energy surface including all degrees of freedom was used in the dynamics calculations. Total energies and gradients were calculated at each time step. The vibrational phase of CH3Cl was generated classically so as to take a temperature of 10 K. The dynamics calculations showed that three reaction channels are concerned with the reaction at a fixed collision energy (E-coll = 4.42 kcal/mol). These are expressed by F- + CH3Cl --> CH3F + Cl- + H2O (channel I), F- + CH3Cl --> CH3F + Cl- (H2O) (channel II), and F- + CH3Cl --> CH3F(H2O) + Cl- (channel III). Channel I is three-body dissociation of each product. In channels II and III, Cl- and CH3F, respectively, are solvated by a water molecule. It was found that the main reaction pathway is channels I and III, while channel II is significantly minor at E-coll = 4.42 kcal/mol. In all channels, the halogen exchange occurs rapidly with very short lifetimes of early and late complexes (i.e., the reaction proceeds via direct mechanism), which is similar to the nonsolvated reaction F- + CH3Cl. The preference of the reaction channels was discussed on the basis of theoretical results.