Two reaction channels occurring in the unimolecular processes were studied by ab initio molecular orbital calculations for the thermal decomposition of isoxazole. These paths constitute the competitive reaction branching from the reaction intermediate. We have failed in finding the transition-state structure for the major reaction channel leading to acetonitrile and carbon monoxide, indicating that these products were formed by the bifurcation due to a dynamical effect on the course of the reaction. We investigated this effect in the reaction theoretically employing the combined approach of the intrinsic reaction coordinate (IRC) and classical trajectory calculations. The IRC calculation indicates that a large curvature of the reaction path exists near the reaction intermediate and that a particular local vibrational mode which couples strongly to the IRC plays a critical role in controlling the consecutive decomposition channel. Classical trajectory calculations proved these findings, which is in conformity with the experimental results of Lifshitz and Wohlfeiler [J. Phys. Chem. 1992, 96, 4505].