The ab initio theoretical calculations on the C-C bond forming stage of the addition of Me(2)Mg to alpha- and beta-alkoxy carbonyl compounds (methoxyacetone, methoxyacetaldehyde, 3-methoxy-2-butanone, 2-methoxypropanal, 4-methoxy-2-butanone, 3-methoxypropanal, and 3-methoxybutanal) were carried out to investigate the reaction mechanism of chelation-controlled carbonyl addition reactions, for which experimental information is scarce. The carbonyl substrate and Me(2)Mg first form a stable chelate complex with 220 kcal/mol exothermicity, which undergoes C-C bond formation via 1,3-migration of the nucleophilic methyl group to the carbonyl carbon with a 6-9 kcal/mol activation energy and then gives rise to a product chelate. Chelate structure was retained throughout the reaction process. The experimental stereoselectivity of the alpha- and beta-chelation-controlled addition has been adequately reproduced. It was found for the first time that the beta-chelation-controlled reaction may proceed via either a stereoselective chair or nonselective boat transition state, the latter being energetically more favored than the former in the present system. The calculations suggested that the C-C bond forming stage is a stereochemistry determining stage as has previously been assumed.