Abstract: This invention is directed to a process for preparing grafted organic polyester of phosphoric acid, incorporated into water-borne coating compositions, reducing remarkably the tendency of aluminum flake pigments to react with the aqueous medium of the compositions, resulting in generation of hydrogen. This can be obtained by grafting the reaction product from the reaction of an orthophosphorus derivative with a compound which contains in the molecule an epoxide group and aromatic or alicyclic group, to a pre-polymer having isocyanate group as an end group and an ethyleneoxide group. The grafted organic polyester of phosphor acid has excellent dispersity with pigments and results in satisfactory adhesion with resin as a binder because it prevents pigments from contacting with water during water dispersing.