An amine oxide is produced by oxidizing a tertiary amine having the formula ##STR1## wherein R.sub.1 and R.sub.2 respectively represent methyl or ethyl group; and R.sub.3 represent a C.sub.8 -C.sub.14 alkyl or alkenyl group in the presence of carbon dioxide and in the presence or absence of an improver selected from the group consisting of ethylenediaminetetraacetic acid, its salts, stanates, polyphosphates, salts of hydroxycaboxylic acid and salts of polycarboxylic acid.