An improved process for preparing a bis [.beta.-(N,N-dimethylamino)ethyl]ether by reacting a bismethohalide of bis [.beta.-(N,N-dimethylamino)ethyl]ether represented by the following general formula: ##STR1## (wherein X denotes a halogen atom), with an aliphatic amine having a boiling point of not lower than 220.degree. C. at atmospheric pressure and having a primary amino group in the molecule thereof. The process comprises the following two steps (a) and (b): (a) a step of distilling out water at a temperature of not more than 130.degree. C. under reduced pressure from a mixture of said bismethohalide aqueous solution and said aliphatic amine to provide a concentration of said bismethohalide to water of not less than 85% by weight, and (b) a step of reacting said bismethohalide and amine in a solid-liquid phase at a temperature of not lower than 140.degree. C.