The　generation　of　boundaries　from　the　triangular　meshes　of　CAD　object　is　very　important　for　successful　surface　segmentation　and　subdivision.　Considering　the　characteristics　of　non-uniformity,　sparseness　and　known　facet　normal　for　STL　model,　which　is　generated　by　a　CAD　system,　a　new　and　efficient　approach　for　extracting　boundaries　on　STL　model　is　presented　in　this　paper.　Boundaries　are　classified　into　sharp　feature　edge　and　non-sharp　feature　edge　embedded　in　the　filleting　and　transitive　region.　The　former　is　detected　based　on　dihedral　edge　angle　(angle　between　facets),　while　the　latter　is　identified　by　analyzing　intrinsic　region　properties　of　triangular　facets　distribution　without　curvature　estimation.　By　means　of　unidirectional　increment　of　feature　edges,　continuous　region　boundaries　are　generated.　Successful　examples　are　presented　to　illustrate　the　feasibility　of　the　proposed　algorithm.