Density functional theory and ab initio calculations were carried out to investigate two probable unimolecular decomposition channels of isoxazole. The calculated structural parameters, dipole moment, and vibrational frequencies of isoxazole are in good agreement with available experimental data. Relative energies of transition states and equilibrium structures evaluated at MP4 and QCISD(T) levels of theory at B3LYP/6-31G** structures indicate that CO and CH3CN, major products of isoxazole thermal decomposition in the temperature range 850-1100 K, are unlikely produced via the mechanism proposed in the experimental and previous theoretical studies. Instead, the mechanism proposed in the current study, isoxazole --> NCCH2CHO --> CH3CN + CO, is more likely the main unimolecular decomposition channel. The mechanism proposed in the experimental study was shown to have a higher activation barrier and is likely responsible for the formation of HCN and H2CCO, minor products of isoxazole decomposition.