N,N-Dimethyl-N-alkylamine or -alkenylamine is prepared by reacting a higher alcohol and dimethylamine by passing hydrogen gas and dimethylamine into a reactor containing a catalyst for the reaction at a pressure of atmospheric pressure to 100 atm. at 150.degree. C. to 250.degree. C. and removing water produced in the reaction as a component of the mixed hydrogen and unreacted dimethylamine gas which is discharged from the reactor, wherein at the point in the reaction at which from 90 to 99% of the higher alcohol has reacted with dimethylamine, one of the following procedures is followed: i) the introduction of only dimethylamine gas into the reactor is stopped, and the reaction is allowed to continue at a temperature 20.degree. to 150.degree. C. lower than the reaction temperature recited above; ii) the introduction of only dimethylamine gas into the reactor is stopped while the temperature is decreased to a level of 20.degree. to 150.degree. C. lower than the temperature recited above, and the reaction is allowed to proceed at the reduced temperature; or iii) the introduction of only dimethylamine gas into the reactor is stopped immediately after decreasing the temperature to a level of 20.degree. to 150.degree. C. lower than the temperature recited above, and the reaction is allowed to continue at the reduced temperature.