Dynamics of H3O following the electron capture by H3O+, which plays an important role in the H2O formation in interstellar cloud, have been investigated by means of ab-initio direct dynamics calculations. The full dimensional potential energy surface, calculated at the HF/6-311+G(d,p) level, was used in the dynamics calculation throughout. The reaction proceeds on the ground state of H3O with zero excess energy at time zero, while H3O is formed by vertical electron capture by H3O+ without structural change. A total of 200 trajectories were run from an initial geometrical configuration generated by ab-initio molecular dynamics calculation for H3O+ at 300 K. The dynamics calculations for H3O showed that two reaction channels are involved in the decay processes of H3O, formed by electron capture by H3O+: one is a dissociation channel in which the reaction proceeds ia a short-lived complex, H3O* (channel 1), and the other channel is one of complex formation in which the reaction proceeds ia a long-lived complex, H3O* (channel 2). The lifetimes of the complexes for channels 1 and 2 are calculated to be about 5–60 fs and >200 fs, respectively, although the branching ratio for channel 2 is negligibly small. The relative translational energy between H2O and H is distributed in the range 150–200 kJ mol−1 with an average of 185 kJ mol−1, which is 90% of the total available energy. The mechanism of the reaction has been discussed on the basis of the theoretical results.