The present invention develops a method of treating metallic or plastic substrates to form mirror finish. After several pre-treatment steps to modify the surface situation of the substrate, whether for metallic or plastic material, to enhance the adhesion of above coating layers, a layer of mirror finish will be applied, with the function of decoration shown with metal appearance and glossy. A changeable topcoat will be coated to protect below mirror finish layer and achieve colorful decoration. All materials and processing steps are environmentally friendly, and involved without expensive facilities.