Abstract: Conventional slope system risk assessment commonly concerns a single failure mode in each numerical realization and their reliability analysis results are difficult to be understood by geotechnical engineers with weak mathematical background. To address the difficulties, this study proposes a slope system risk estimation method based on region probability incorporating multiple failure modes in the slope system. First, the concept of region probability is provided. The method for representing slope stability quantitively with region probability is then established. The formula for estimating of slope system risk based on region probability is derived and its applicability is discussed. Monte Carlo simulation (MCS) is then employed to calculate the region failure probability. An undrained cohesive slope cut is used to demonstrate the performance of the proposed approach. The results indicate that the region failure probability provides a simple, straightforward and visual tool for the identification of key dangerous regions of the slope, which in turn can give insights into the design of countermeasures for slope reinforcement and risk mitigation. In addition, the region failure probability-based risk assessment method can properly estimate the slope system risk and effectively avoid sophisticated correlations among the sliding blocks and the factor of the large number of potential slip surfaces. Conventional risk assessment methods may lead to a significant underestimation of slope system risk, which may incur an under-design of slope and is detrimental to the slope system. The slip surface associated with the largest risk may not be the slip surface associated with the largest failure possibility. The slip surface associated with the largest risk deserves the same attention as that associated with the largest failure possibility in the slope design and reinforcement.