PURPOSE: To alkylate the side chain alkyl group of alkyl aromatic hydrocarbon at a high yield, by containing copper, magnesium or the like in a zeolite catalyst subjected to ion exchange with an alkali metal as essential components.

CONSTITUTION: As a catalyst composition for alkylating the side chain of alkyl aromatic hydrocarbon such as toluene, one or more of copper, magesium or the like is contained in zeolite subjected to ion exchange with an alkali metal such as potassium or rubidium as an essential component. The resulting catalyst alkylates the side chain alkyl group of alkyl aromatic hydrocarbon at a high yield.