Abstract: There is disclosed a non-aqueous electrolytic aluminum plating bath composition comprising an aluminum halide and a nitrogen-containing heterocyclic onium halide compound and containing an additive selected from the group consisting of an inorganic halide compound, aromatic aldehyde, ketone or carboxylic acid, an unsaturated heterocyclic compound containing more than one nitrogen atom, an unsaturated heterocyclic compound containing a sulfur atom, an aromatic hydrocarbon compound containing a sulfur atom, an aromatic hydrocarbon compound containing an amino group and an aromatic amine; and further optionally an organic polymer. The bath composition is easy in maintenance, has good covering power, and enables smooth plating with low current density.