More than 76% of the reservoirs in Korea are considerably aged. In addition, natural factors such as heavy rain, earthquake, and the heterogeneity of material properties result in the risk of reservoir collapse. However, the existing stability evaluation methods could not consider decay factors. Therefore, in this study, the stability evaluation is performed by supplementing the limitations of the current safety management technology on the risk of reservoir collapse and reflecting the characteristics of the material. For stability evaluation using numerical analysis, the standard sectional shape and material properties of the reservoir are applied based on design criteria, and the width and depth of the ground are applied following sensitivity analysis. A stability evaluation considering the effects of heavy rain and earthquake is performed. And, the various models of the safety factor are proposed. Also, the distribution of the safety factor and the probability of failure is determined following probabilistic analysis considering the heterogeneity of the material properties. Then, compared existing study. In addition, stability evaluation is performed for actual reservoirs in Gyeongju and Yeongcheon by utilizing the variation model of the safety factor. Results show that the safety factor is decreases as the height and water level of the reservoir increase. And the probability of failure is lower than that of the previous study, considering the heterogeneity of the material properties. Lastly, by applying the variation model of the safety factor to the actual reservoir, the lower the height of the reservoir, the higher the level of rising for the same heavy rain, so the safety factor tends to decrease. In conclusion, the main purpose of this study is to predict the stability of disasters and to evaluate the reasonable probability of failure. Based on the results of the study, the necessity of reinforcing the reservoir slope is required.