The　Numerical　Manifold　Method　(NMM)　has　been　considered　as　an　effective　analysis　method　for　geotechnical　problems.　A　vector-sum-based　numerical　manifold　method　(VSNMM)　combined　with　a　pattern　search　algorithm　(PSA),　termed　VSNMM-PSA,　is　further　proposed　to　assess　the　stability　of　slopes.　In　this　numerical　method,　PSA　is　applied　for　the　determination　of　failure　surface　location.　With　the　stress　field　of　the　slope　obtained　from　a　NMM-based　elastoplastic　calculation,　the　vector　sum　method　(VSM)　is　used　to　predict　trial　safety　factors　for　a　series　of　trial　failure　surfaces　(TFSs)　during　the　failure-surface　determination　process.　Stability　analyses　about　three　standard　slopes　are　investigated　with　the　proposed　VSNMM-PSA　model.　Numerical　results　indicate　that　slopes＇　safety　factors　and　failure　surfaces　can　be　accurately　determined　through　the　proposed　VSNMM-PSA　method.　The　proposed　VSNMM-PSA　method　provides　an　effective　tool　to　evaluate　slopes＇　stability,　and　may　improve　their　designs.