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Ozonolysis of unsaturated ketones is a common atmospheric chemical process that plays a significant role in controlling the atmospheric budget of OH and O 3 , organic acids, and secondary organic aerosols (SOA). In this work, the detailed reaction mechanism and rate coefficients for the reactions of O 3 with two unsaturated ketones, 3-methyl-3-buten-2-one (MBO332) and 3-methyl-3-penten-2one (MPO332), were investigated by using density functional theory (DFT) and Rice−Ramsperger−Kassel−Marcus (RRKM) theory. The results indicate that the major products are butanedione and formaldehyde for MBO332, and butanedione and acetaldehyde for MPO332. Possible reaction mechanism and thermodynamic parameters of some complex stable Criegee intermediates (SCIs) RR′COO were also be investigated in this study. Some organic peroxides can be regarded as the main products for the further reactions of SCIs. The rate constants calculated with O 3 are 2.59 × 10 −16 cm 3 molecule −1 s −1 and 2.28 × 10 −16 cm 3 molecule −1 s −1 for MBO332 and MPO332 at 298 K and 1 atm. The total rate constant is negatively correlated with temperature (200−400 K) and positively correlated with pressure. The atmospheric half-lives of MBO332 and MPO332 based on O 3 are estimated.