This study seeks to investigate the effect of joint surface damage on the shear strength of two natural rocks, namely sandstone and argillite. A series of shear box tests were performed on the jointed rock specimens with different joint roughness coefficients (JRC). The joint surface roughness of each rock specimen was estimated by means of Barton’s comb before and after the shear test as well as it was obtained experimentally using the measured peak shear stress. The laboratory data indicated that some damage of joint surface occurred during shearing, which affected the overall shear strength of the jointed rock specimens. The damage coefficient (M) initially introduced by Baron and Chubey (1977) was modified so that it can be used to estimate the joint surface damage of the tested rocks; that is, no or small damage may occur when M<1 while considerable damage to the joint surface can be expected when M>1. Due to the inhomogeneous of the rock samples, there are two modulus presented for the damage coefficient. This study seeks to find a more accurate relation between the JRC value and the damage coefficient based on the Barton and Choubey theory.