Surface treatment is used for the manufacturing process of automobile parts, and it includes many purposes and kinds. For example, lubricity is demanded from processed material in the forging process, and seizure resistance is necessary for the dies. In addition, wear resistance, fatigue endurance and corrosion resistance are required to automobile parts. Surface treatment includes solid phase, liquid phase, gas phase and plasma process. In this section, we introduce surface treatment using the liquid phase method such as the chemical conversion coating and salt bath nitriding.