The basic aim of this study is to develop a reliability-based methodology for the practical design of rock slopes. The advanced first-order second-moment (AFOSM) approach is adapted as;the reliability assessment model. In the reliability-based design methodology, the first step is the modeling and quantification of uncertainties associated with the design parameters. Since the safety of a rock slope is most sensitive to the shear strength of discontinuities, the uncertainty analysis is focused on the parameters associated with the discontinuity shear strength. These uncertainties are modeled and analyzed within the framework of AFOSM method and all relevant information is utilized in their quantification. A wedge failure case is considered for the implementation of proposed design methodology in which a comprehensive uncertainty analysis for the shear strength of discontinuity planes are carried out and the mean and variance of the basic variables are quantified. Based on the AFOSM reliability technique, reliability index and probability of failure for various slope angle and slope height combinations are computed and safe design slope angles for every slope height are selected according to a specified acceptable level of reliability that is determined according to the previous recommendations in the literature.