Provided is a method of forming a metal interconnection line on a flexible substrate, wherein the method includes: coating a hard mask layer on at least one surface of the flexible substrate, followed by performing photolithography thereon to form a predetermined hard mask pattern; etching a portion of the flexible substrate by using the hard mask pattern as a mask to form a trench; plasma treating the inside of the trench by using a treatment gas for pre-treating the flexible substrate; coating a seed layer inside the trench; removing the hard mask pattern; and filling the inside of the trench coated with the seed layer with metal. A metal interconnection line formed by using the method may have a strong adhesion force with respect to the flexible substrate.