PURPOSE: To efficiently prepare an alkyl ketene dimer by the dehydrohalogenation of a linear fatty acid halide with a cyclic tetiary amine in an inert solvent.

CONSTITUTION: An alkyl ketene dimer is obtained by reacting a 8-22C saturated or unsaturated linear fatty acid halide or its mixture with a cyclic tertiary amine of formula (R is 1-7C substituted or unsubstituted alkyl; n is 2 to 10), preferably Nmethylpyrrolidine or N-methylpiperidine in an inert solvent at up to 75°C, favorably 55 to 65°C, mechanically separating (preferably centrifugally separating) crystals of tertiary amine hydrochloride salts from alkyl ketene dimer dissolved in the reactive solvent, and subsequently removing the solvent and residual tertiary amine by distillation. As the method of reaction it is favorable to add dropwise a fatty acid halide dissolved or undissolved in an aliphatic hydrogen carbide to the tertiary amine solution in an aliphatic hydrogen carbide solvent.