Dual-level dynamics calculation with variational transition state theory including multidimensional tunneling has been performed on the isomerization reaction of cyclic ozone -> normal (open) ozone, which was believed to be the stability-determining reaction of the elusive cyclic ozone molecule under thermal condition. The high-level potential energy surface data were obtained from the calculation using the MRCISD+Q theory with the aug-cc-pVQZ basis set, while the low-level reaction path information was obtained using the hybrid density functional theory B3LYP with the cc-pVTZ basis set. the calculated results showed very significant tunneling effects below 300 K (a factor of similar to 200 at 300 K and over 10(7) at 200 K). Because of the strong tunneling effects and the potential energy surface crossing of a 1A(1) and 1A(2) states, the isomerization reactions were found to be significantly faster than previously believed. The half-life of the cyclic ozone was estimated only similar to 10 s at 200 K and similar to 70 s below 100 K, which might partly explain the unsuccessful attempts for its experimental indentification. The kinetic isotope effects (KIEs) for various (18)O substitution reactions were also calculated as function of temperature and were as high as 10 at very low temperature. Because of the large KIEs, the experimental identification of the cyclic (18)O(3) seems more promising.