Abstract: Based on the accurate calculation formulae of safety factor,the minimum reliability index(βmin) and the minimum safety factor(Fsmin)(with the reliability index βFmin) of simple soil slopes are searched directly.The difference between the safety factor method and the reliability index method,and the relationship between βmin and βFmin are investigated.The results reveal that the traditional slices method(GLE method) is conservative,and it may give an unrealistic evaluation,especially for the circle arch surface failure with resisting soil part.Considering the probability distribution type and variable characters of strength parameters,the safety factor(Fsi) and failure probability of each slip surface are calculated by use of Monte-Carlo method.And the reduced safety factor is given by FsRi=Fsi(1-Pfi).A slope stability evaluation system is founded using the minimum reduced safety factor FsR=min(FsRi).Several typical examples are studied.The results show that the evaluation system is in good agreement with the dual index system by others.