Criegee intermediate is believed to play an important role in the atmospheric chemistry.Because of its short life and the difficulty in experimental study, we carried out ah initio calculations on the thermochemistry of the Criegee involving reactions in this study. Thermochemistrydata of reaction enthalpies and Gibbs free energies for four different stable structures of the Criegeeintermediates (singlet CH2OO ①1 A1 in C2v, triplet CH2OO ②3B1 in C2v, singlet CH2OO ③1A’ in Cs and triplet CH2OO ④ in C1 symmetry) involved in some of the gas-phase reactions were calculated at the standard Gaussian-2 [G2(MP2) and G2] and a modified G2, G2(fu1)[10],levels of theory. Relative energies among those Criegees and formic acid were compared. Chemical reactions incltlde the formation of Criegees, re-arrangement from Criegee to formic acid, dissociations (producing CH2(3B1)+O2, CH2(1A1)+O2, CO2+H2, CO2+2H, CO+H2O, OH+HCO) andthe reactions between Criegee and NO/H2O. Standard equilibrium constants for some reactions were investigated and may be obtained for all of the rest reactions involved in this study by the standard Gibbs free energies. It is shown that the formation of Criegee①-④ by ethylene and ozone, the re-arrangement from any Criegee to formic acid, the dissociation in producing CO2+O2and CO+H2O and the reactions between any Criegee and NO/H2O are all favourable thermodynamicaly. The dissociation in forming CO2+2H and OH+HCO is less favourable. While the dissociation in forming carbene (either in 3B1 or 1A1 state) is not allowed by values. Standard enthalpies of formation at 298 K for the four Criegees were predicted at the G2(ful) level of theory. Each value is the average value from ten of the above reactions and they are -4.3, 74.8,98.9 and 244.6 kJ mol-1 at the G2(ful) level for Criegee ① to Criegee ④, respectively. In addition, tile standard enthalpy of formation at 298 K for HOCH2OOH is further predicted to be -315.6 kJ mol-1 at the G2(MP2) level. Criegee intermediate is believed to play an important role in the atmospheric chemistry. Butcause of its short life and the difficulty in experimental study, we carried out ah initio calculations on the thermochemistry of the Criegee involving reactions in this study. Thermochemistrydata of reaction enthalpies and Gibbs free energies for four different stable structures of the Criegee intermediates (singlet CH2OO ① 1 A1 in C2v, triplet CH2OO ② 3B1 in C2v, singlet CH2OO ③ 1A ’in Cs and triplet CH2OO ④ in C1 symmetry) involved in some of the gas-phase reactions were calculated at The standard Gaussian-2 [G2 (MP2) and G2] and a modified G2, G2 (fu1) [10], levels of theory. Relative energies among those Criegees and formic acid were compared. Chemical reactions incltlde the formation of Criegees, re -arrangement from Criegee to formic acid, dissociations (producing CH2 (3B1) + O2, CH2 (1A1) + O2, CO2 + H2, CO2 + 2H, CO + H2O, OH + HCO) and these reactions between Criegee and NO / H2O. Standard equilibrium const ants for some reactions were investigated and may be obtained for all of the rest reactions involved in this study by the standard Gibbs free energies. It is shown that the formation of Criegee①- ④ by ethylene and ozone, the re-arrangement from any Criegee to formic acid, the dissociation in producing CO2 + O2 and CO + H2O and the reactions between any Criegee and NO / H2O are all favourarable thermodynamicaly. The dissociation in forming CO2 + 2H and OH + HCO is less favourrable. While the dissociation in forming carbene ( either in 3B1 or 1A1 state) is not allowed by values. Standard enthalpies of formation at 298 K for the four Criegees were predicted at the G2 (ful) level of theory. Each value is the average value from ten of the above reactions and they are -4.3, 74.8, 98.9 and 244.6 kJ mol-1 at the G2 (ful) level for Criegee ① to Criegee ④, respectively. In addition, the tile standard enthalpy of formation at 298 K for HOCH2OOH is further predicted to be -315.6 kJ mol-1 at the G2 (MP2) level.