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A new concept of critical shear strain ετcritical of rock joint under cyclic loading is presented, and the role of ετcritical in evaluating the sliding potential of rock joint is highlighted. A series of cyclic triaxial tests was conducted on a cylindrical rock joint specimen with a replicated rough surface representing a joint roughness coefficient JRC value of 12.6 oriented at 60° with respect to the horizontal plane. The experimental results indicate that the onset of instability of rock joint is suppressed with increase in confining pressure and number of loading cycles N until the normalized shear deformation increases beyond a threshold value of ετcritical. Generally, the critical strain of most rock types is considered in the proximity of 1% under small strain conditions [36–37], however, in this study, the critical strain concept is extended to the domain of rock joints, and a semi-empirical model to more rigorously quantify the critical shear strain (ετcritical) of rock joint is suggested considering the effect of joint roughness coefficient JRC, cyclic loading amplitude, and the number of loading cycles N. Also, a rational classification of Joint Sliding Potential (JSP) based on the ετcritical and normalized total shear strain εθN of rock joint is proposed to characterize the cyclic loading induced sliding instability of a rock discontinuity.