Twelve unimolecular reaction channels of the CH3CHClO radical have been studied using the ab initio G2-(MP2,SVP) method. The calculations detailed three kinds of mechanisms, i.e., bond scission, intramolecular three-center elimination, and isomerization. On the basis of eb initio data, we performed multichannel (up to eight channels) RRKM calculation and numerical master equation analysis. The energy-specific rate constants, k(E), and the thermal rate constants, k(T,P), were obtained. The three-center elimination of HCl from CH3CHClO was shown to be the dominant decomposition pathway. This finding provides theoretical evidence for the previous experimental result. The implication of our results was discussed in terms of understanding the atmospheric fate of the chemically activated or thermally balanced CH3CHClO radicals.