A process for producing a monoalkenyl aromatic hydrocarbon compound which comprises alkenylating a side chain of an aromatic hydrocarbon compound having at least one hydrogen atom bonded at an .alpha.-position of the side chain (such as xylene) with a conjugated diene having 4 or 5 carbon atoms (such as butadiene) in the presence of a catalyst comprising a mixture obtained by heat treating metallic sodium together with a mixture of a zirconium oxide and a potassium compound in an inert gas atmosphere. An industrially useful monoalkenyl aromatic hydrocarbon compound is produced in a high yield at a low cost with enhanced safety from a specific aromatic hydrocarbon compound and a specific conjugated diene compound.