Surface treatments of polyethersulfone (PES) films were performed using the magnetized plasma generated by low-frequency (60Hz) power for applying them to the flexible substrates of solar cells and electronic displays. The contact angle between the water droplet and PES films was observed to change from 80 degrees to 30 degrees after the plasma treatment. This indicates that the surface of the PES films changed from hydrophobic to hydrophilic. The surface roughness of the films did not change after the plasma treatment, and this indicates that the magnetized plasma did not damage the surface of the films. The trapping of energetic particles by the magnetic field is thought to be responsible for this result. It is concluded that the plasma treatment by using the magnetized plasma is useful for the surface modification of the polymeric materials suitable for flexible substrates of solar cells and electronic displays.