A method of modifying a polymeric material whereby the polymeric material is modified to give a highly durable surface without decreasing its strength. Namely, a polymeric material is modified by the method comprising a combination of an impregnation step, an activation step, a step of grafting a monomer and a step of a treatment with a hydrophilic polymer. Thus, the hydrophilic nature, adhesiveness, and so forth of polymeric materials such as polyolefins can be improved without causing any decrease in the practical strength. The thus obtained polymeric materials are usable in articles with a need for high water absorptivity and high adhesiveness, for example, water-absorbing materials and water-retaining materials for medical, sanitary, cosmetic articles or supplies, agricultural materials, synthetic papers, filters and fiber products for clothes, for improving adhesiveness of composite materials and the like. Among all, these materials are particularly usable in dentition orthodontic appliances, writing implements and battery separators.