Abstract: Based on the asperity of a rough joint occurring on many scales，Joint roughness coefficient (JRC) is presented using the elongation rate (Er) and degree of the undulation(wd），and their relationship is established through the value of standard profiles suggested by ISRM. The elongation rate (Er) is used to show the roughness character offered by secondary asperity，while the degree of an undulation (wd) is used to reflect the roughness property at the first order. On the other hand，in virtue of the difficulties in calculating the value of curve length in engineering practice，this paper provides the program for obtaining the value of curve length based on the digital image of rock joints. It would be helpful for applying the method to describe the JRC in engineering practices.

Key words: Joint roughness coefficient (JRC), Degree of the undulation, Elongation rate, Digital image