The mechanism of ozonide formation has been being focused on the study of ozonization of alkenes. Criegee's three-step mechanism has been widely accepted. The firsst step is the formation of a primary ozonide. In most cases, it is a concerted cycloaddition of alkene to ozone. The second step is fragmentation of the primary ozonide into a carbonyl oxide and a carbonyl compound. Carbonyl oxide is the core of Criegee's mechanism. The ratio of carbonyl oxide to carbonyl compound depends on the structure of the alkene. The third step is recombination of the carbonyl compound with carbonyl oxide. Besides newly-formed carbonyl compound, other already existed carbonyl group can also react with carbonyl oxide. Carbonyl oxide can also react with compounds bearing active proton, such as water, alcohols, organic acids and amines. Literature of the Criegee's mechanism is reviewed. Reaction process, selectivity and stereochemistry of each step are discussed on the basis of the related literature.

Key words: OZONIZATION, OXIDE, OXIDATION, ALKENE, CARBONYL GROUP, CARBONYL COMPOUNDS