The tunneling effects in the molecular inversion of aziridine are investigated. A full-dimensional potential was constructed based on nb initio results and used to study the effect of vibrational excitations on tunneling. Using the semiclassical approach that incorporates tunneling into classical trajectory calculations, it is found that excitations of all the modes except the reaction coordinate have negligible effects on tunneling. This allows accurate thermal rate calculations carried out by using a one-dimensional model. (This is in accord with the conclusions of the work by Rom er nl. [Chem. Phys. Lett. 204, 175 (1993)] and by Smedarchina er al. [J. Chem. Phys. 102, 7024 (1995)].) A one-dimensional Weutzel-Kramers-Brillouin (WKB) formula was employed to obtain the level splitting and was found to be very accurate based on comparisons with the quantum results. Furthermore, the calculated thermal rates are in good agreement with the experimentally measured values. The results provide insight into the reaction mechanism and explanations for the experimental findings of non-Rice-Ramsperger-Kassel-Marcus (RRKM) behavior and incoherent tunneling.