PURPOSE: To provide a method for continuously preparing quat. ammonium salts in a high conversion ratio and high selectivity by reacting a tert. amine with an organic halide on the heterogeneous catalyst composed of metal oxides having Lewis acid sites.

CONSTITUTION: The quat. ammonium chloride is prepared by continuously reacting a long chain tert. amine with an alkyl chloride in a solvent such as isopropanol in the presence of a heterogeneous catalyst composed of metal oxides having Lewis acid sites (especially, oxides such as silica, zirconia or titania, dioxide of hafnium, alumina or mixture of them). The process is especially suitable for preparing the quat. ammonium chloride of formula III by reacting the tert. amine of formula I (R1 is a 8-22C aliphatic group; R2, R3 are each a 1-22C aliphatic group) with an alkyl chloride of formula II (R4 is methyl, ethyl, benzyl or the like).