The　algorithm　of　3D　geometric　shape　discriminant　is　presented,　and　a　method　of　form　error　evaluation　based　on　discriminant　characters　is　proposed　to　realize　automatic　geometric　form　error　evaluation.　The　method　can　determine　the　measured　shape　type　on　the　basis　of　the　invariant　feature　of　the　measurement　point　set　and　can　evaluate　the　form　error　automatically　in　terms　of　minimum　zone　criterion　without　depending　on　CAD　model　and　the　parameters　of　measured　workpiece.　The　evaluation　mathematical　model　and　initial　parameters　of　the　algorithm　are　determined　according　to　the　discriminant　characteristics.　Case　study　on　plane,　sphere　and　cylinder　explains　the　implementation　process　of　this　method.　The　numerical　simulations　were　carried　out,　which　verifies　that　the　discriminant　algorithm　can　accurately　determine　the　shape　of　measurement　point　set,　and　provide　initial　parameters　and　accurate　model　for　form　error　evaluation.　Actual　measurement　data　processing　results　show　that　this　method　has　high　accuracy　and　versatility,　and　is　a　new　form　error　measurement　and　evaluation　mode.