The study of friction is very important in rock mechanics. Friction plays an important role in the sliding process along the discontinuous plane at various scales. According to Soda (1971), the friction phenomena have been studied as the resistances to sliding of solid materials by many investigators, particularly Leonardo daVinci (14521519), Amonton (1663-1705), and Coulomb (1736-1806). The modern knowledge of frictional phenomena is mainly attributed to the work of Bowden and Tabor (1950, 1964).