Region　segmentation　involves　in　breaking　a　polygonal　mesh　into　＇meaningful＇　patches　with　characteristic　geometrical　property　respectively.　Considering　the　characteristics　of　non-uniformity,　sparseness　and　known　facet　normal　for　STL　model　from　CAD　system,　an　efficient　approach　for　region　segmentation　is　put　forward.　The　algorithm　contains　two　steps:　extracting　region　boundaries　and　segmenting　regional　meshes.　Region　boundaries　are　classified　into　sharp　feature　edge　and　non-sharp　feature　edge.　The　former　is　detected　based　on　dihedral　edge　angle　(　angle　between　facets),　while　the　latter　is　identified　by　analysing　intrinsic　region　properties　of　triangular　facets　distribution　without　curvature　estimation.　By　means　of　unidirectional　increment　of　feature　edges,　continuous　region　boundaries　are　generated.　Finally,　we　withdraw　＂meaningful＂　patches　from　STL　model　based　on　advancing　front　algorithm　depending　on　the　boundaries.　Examples　are　given　to　show　the　feature　edges　detection,　boundaries　generation　and　region　segmentation　based　on　the　boundaries.