To　study　the　mechanism　of　forming　limit　enhancement　in　sheet　incremental　forming,　a　cold-rolled-sheet　square　frustum　was　incrementally　formed　on　a　numerical　controlled　milling　machine.　The　change　of　the　micro-structure　in　the　process　was　studied　by　using　metallographic　microscope　and　scanning　electron　microscope(SEM),　and　compared　with　normal　stamping　process.　It　was　found　that　cold-rolled　sheets　can　be　incrementally　formed　to　produce　components;　under　sump　lubrication,　the　incremental　deformation　was　cold　since　the　heat　produced　by　friction　was　not　enough　to　change　part＇s　microstructure;　the　microstructure　told　that　the　contact　part　of　sheet　was　smashed　and　uniformly　enlarged　later,　which　demonstrated　that　the　deformation　in　incremental　process　is　local,　alternative,　and　little　increment,　and　the　uniform　distribution　of　deformation　enhances　the　forming　ability.　©,　2015,　Beijing　Res.　Inst.　of　Mechanical　and　Elec.　Technology.　All　right　reserved.