To provide a method for alkylating an aromatic compound by reacting the aromatic compound with an alkyl mesylate in the presence of a rare earth element triflate as a catalyst, capable of alkylating the aromatic compound by the new method.

This method for alkylating an aromatic compound (for example, benzene or naphthalene) comprises reacting the aromatic compound with an alkyl mesylate (preferably a secondary alcohol methanesulfonate ester such as cyclopentanol methanesulfonate ester). Therein, a rare earth element triflate compound (for example, a lanthanide triflate) is used as a catalyst. The reaction may be carried out under heating. The rare earth element triflate compound is obtained by reacting a rare earth element oxide with trifluoromethanesulfonyl chloride. The catalyst is preferably used in an amount of 5-20 mol.% based on the amount of the alkyl mesylate.