The basic aim of this study is to formulate a probabilistic method by which sources of uncertainties can be taken into account and systematically analyzed in dealing with the rock slope stability problem. For this purpose, the advanced first-order second-moment (AFOSM) method and the Monte Carlo simulation (MCS) procedure are used to model the plane failure problem in rock slopes. The modeling of plane failure by the AFOSM and the MCS methods requires the understanding of the basic mechanism of plane failure, generation of a failure function, identification of probabilistic and deterministic parameters and assessment of the uncertainties involved in the probabilistic parameters. The peak friction angle of the rock discontinuity in the failure function is treated as the main uncertain parameter. In both of the methods, the uncertainties associated with the peak friction angle of rock discontinuities are assessed based on a model developed within the framework of first-order second-moment approach. The probability of failure values from both MCS and AFOSM methods are evaluated for the normal and lognormal basic variables considering the stability of slope of Kanmantoo Mine in South Australia. A sensitivity analysis, based on AFOSM model with lognormal basic variables, is performed in order to investigate the influence of the variability in peak friction angle on the probability of failure results by considering various values for the coefficient of variation.