In geotechnical problems, uncertainty and inherent variability in soil strength parameters is undeniable. A common and effective way of considering uncertainty isMonte Carlo simulation method. In this method, the primary objective is to obtain the reliability index and probability of failure for the critical slip surface, or in other words, deterministic slip surface. However, the slip surface with the minimum factor of safetymay not be the surface with minimumreliability index or the highest probability of failure. In the present research work, this problemwill be scrutinized on an earth Dam. Furthermore, the global reliability index and probability of failure of the damwill be introduced. Specific gravity, seismic pseudo static coefficient and soil strength parameters including cohesion and friction angle are taken into account as the sources of uncertainty. The results show that the common approach of calculating the reliability index may not be conservative.