W18O49　has　exceptional　electronic　and　electro-chromic　properties　and　has　been　synthesized　in　various　morphologies　for　applications　in　nanodevices.　Nevertheless,　the　controlled　growth　of　nanostructured　W18O49　with　desirable　morphology　is　not　yet　achieved,　and　the　growth　mechanism　is　not　yet　clear.　In　this　work,　we　synthesized　ultra-long　single-crystalline　W18O49　micron-rods　via　heating　W　matrix　under　an　atmosphere-Ar　flow,　and　systematically　investigated　the　nucleation　and　growth　by　detailed　micro　structural　characterization.　The　presence　of　WO2　interlayer　above　the　W　matrix　was　reported　for　the　first　time,　which　was　confirmed　to　play　a　key　role　in　the　epitaxial　nucleation　of　W18O49.　Extensive　annealing　led　to　the　growth　of　WO2　grain　and　W18O49　nanostructures　into　micron-rods,　which　suggests　that　W(18)O(49)diameter　is　controlled　by　WO2　grain　size.　This　study　is　helpful　in　the　design　of　W18O49　nano　structures　for　various　applications.　(C)　2018　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.