A method of modifying a polymeric material which comprises the steps of activation-treatment and a hydrophilic polymer-treatment, or comprises the steps of activation-treatment, a hydrophilic polymer-treatment, and monomer grafting in this order, or comprises the step of a solvent-treatment followed by these steps. Thus, the polymeric material, e.g., polyolefin, is improved in hydrophilicity, adhesion, etc. without lowering the practical strength thereof The polymeric material thus improved in adhesion and other properties can be used in many applications where water absorption and adhesion are required, such as an absorption material, e.g., a wiping/cleansing material, a water retention material, a material for microorganism culture media, a separator for batteries (or cells), a synthetic paper, a filter medium, a textile product for clothing, a medical/sanitary/cosmetic supply, and reinforcing fibers for composite materials.