The assessment of the safety level of natural slopes, road cuts, embankments and levees require consideration of uncertainties and variability in material properties. In this study, for a number of slope geometries, including a real-life landslide case, probability of failure (PF) and the most critical failure surface are investigated with and without cross-correlation of shear strength properties. Slopes having different traditionally-defined factor of safety (FS) levels are studied. The uncertainty of soil properties are considered by different levels of coefficient of variation (COV). Limit equilibrium method is used for slope stability analyses and geotechnical material properties are considered to have normal statistical distribution. The results of this analyses show that the PF and the critical failure surface is significantly influenced by the COV level, the consideration of cross correlation of shear strength parameters, and by the traditional FS level of the slopes. The inverse relation between FS and PF is demonstrated to be nonlinear and the COV level has significant effect on this relationship. Results indicate that the deterministic slope stability analyses resulting in a single FS value is no longer sufficient to evaluate the safety of a slope in geotechnical engineering, and that the deterministic critical failure surface with minimum FS value is not always the most critical slip surface. The results presented in this study could be useful for further understanding of probabilistic slope stability and the effects of soil variability/uncertainty, with the aim of better geotechnical risk evaluation and communication.