The　numerical　manifold　method　(NMM)　has　been　used　to　solve　geotechnical　problems　with　valuable　results.　In　this　paper,　a　numerical　model　based　on　the　NMM　is　proposed　to　investigate　two　primary　concerns　in　slope　stability　analyses,　i.e.,　critical　slip　surface　(CSS)　determination　and　safety　factor　(Fs)　calculation.　In　our　proposed　numerical　model,　the　optimization　algorithm　of　MAX-MIN　ant　colony　(MMACOA),　which　is　considered　to　be　an　algorithm　with　the　best　performance　for　such　optimization　problems,　will　be　used　to　determine　CSS.　In　the　process　of　determining　the　CSS,　the　trial　safety　factor　for　a　trial　slip　surface　is　calculated　with　the　vector　sum　method　(VSM)　according　to　the　stress　field　produced　by　an　NMM　analysis.　Accuracy　of　the　numerical　model　is　validated　with　three　typical　examples.　The　numerical　model　has　great　potential　in　the　evaluation　of　stability　of　real　slopes.