To prevent the environmental pollution, water hydraulic systems are replacing oil systems. However, the systems have some technical problems, including tribology and corrosion. Many researches have demonstrated that diamond-like carbon (DLC) films have excellent tribological properties under unlubricated conditions. We have developed DLC films covered with Si-doping DLC layer using a plasma-enhanced CVD technique and evaluated their tribological properties in water environment. They had a low friction coefficient of less than 0.1 and a very small wear rate of 5×10^-8 mm³/Nm. Moreover, they showed good adhesion to the substrate.