Abstract: In order to apply reliability theory in cutting loess slope design, 510 natural critical loess slope sections in the eight regions in loess plateau were investigated and the relationship between the slope heights and widths was regressed in each region. Considering the factor of safety as 1.0 with the profile of the regressed relation, comprehensive shear strength parameters c、 are reversed against different slope heights. Morgenstern-Price method was used to build limit state equation as well as Monte-Carlo method was used to simulate probability of failure against the reversed shearing strength and statistical results from 79groups of c、 coefficient of variations. Then the relations of the loess high slope probability of failure, factor of safety and coefficient of variations are showed systemically and the reliability criteria for loess cutting slope is proposed. The failure probability of cutting loess slopes should lower than 8.0%for express way and first grade highway as well as within 15.0% for second grade highway and lower ones. The factor of safety and the probability of failure have their own advantage respectively. Therefore, combining both of them is the better choice in assessment of slope stability in loess area where geo-hazards occur frequently.