According to the present invention, by using 4-halogeno-3-hydroxybutanamide as a substrate in quaternary amination reaction with trialkylamine which is an important step in betaine (such as carnitine) preparation processes, it becomes possible to reduce the production of crotonic acid derivatives (the major by-product) greatly compared to conventional processes. Consequently, it becomes possible to prepare a betaine, such as carnitine, at a high yield. The present invention also relates to a process for preparing a betaine represented by formula (1) below, comprising a step of quaternary aminating an amide represented by formula (2) below: ##STR00001## wherein A.sup.1, A.sup.2 and A.sup.3 individually represent a C.sub.1-C.sub.20 hydrocarbon group which may have a substituent(s); and X.sup.1 is a halogen atom.