The optimum design of a pit requires the determination of the most economic pit limit which normally results in steep slope angles as in this way the excavation of waste is minimized. In general, as the slope angle becomes steeper, the stripping ratio is reduced and the mine economics improves. However, these benefits are counteracted by an increased risk to the operation. Thus the determination of the acceptable slope angle is a key aspect of the mining operation. The difficulty in determining the acceptable slope angle stems from the existence of uncertainties associated with the stability of the slopes. The uncertainty might be due to a random variability or lack of aspect knowledge. Field data collection and site investigations are used to reduce these uncertainties and to define the inherent natural variability. Effects of geomechanical property uncertainty on probability of slope failure are significant. In this paper Monte Carlo Simulation (MSC) method was selected to use for incorporating geomechanical property uncertainty in examining the probability of failure of the slope in Sungun Copper Mine (SCM) in Iran and Aitik Copper Mine (ACM) in Sweden.