The surface of polytetrafluoroethylene (PTFE) was modified hydrophilically by the bombardment of positive ions formed by introducing water into an ion source (H₂O ion bombardment). The contact angle of water on the PTFE was changed from 120° to 0° by H₂O ion bombardment with 0.6×10¹⁶ ions/cm². In this range, the surface roughness of the PTFE was not changed. XPS analysis showed that OH-groups were created on the surface. Copper thin film was deposited on the PTFE surface after H₂O ion bombardment, and the bonding strength between the Cu thin film and PTFE increased in pro-portion to the quantity of ion bombardment. At H₂O ion bombardment with 1.2×10¹⁶ ions/cm², the bonding strength was 12MPa. Our experimental results suggest that the oxide layer of the Cu thin film and the OH-groups of the PTFE surface are connected by hydrogen bridges.