The invention provides a novel method for modifying surface properties, e.g. liability to surface bleeding of plasticizers and other additives and affinity with water, of shaped articles of polymeric materials such as vinyl chloride-based resins and silicone rubbers by the treatment with low temperature plasma of a gas. The plasma treatment in the inventive method is carried out in two steps with the first step carried out in an atmosphere of oxygen and the second step carried out in a non-oxidizing gas such as argon and carbon monoxide. Different from the conventional methods of plasma treatment which suffer from poor reproducibility and low effectiveness, the inventive method can give good reproducibility and high effectiveness presumably owing to the removal of the surface stain in the first step plasma treatment.