A　series　of　transmission　electron　microscopy　experiments　were　carried　out　for　studying　the　structure　and　chemistry　thermal　stability　of　thermoelectric　material　single　crystalline　SnSe　with　Na-doping.　With　spherical　aberration-corrected　scanning　transmission　electron　microscopy　and　high　sensitivity　energy　dispersive　X-ray　detector,　both　of　the　atomic　structural　and　compositional　defects　of　Na-doped　SnSe　were　uncovered.　Lamella-shaped　defect　with　depleted-Sn　but　rich-Na　was　captured.　In　situ　heating　experiments　up　to　450　degrees　C　revealed　a　thermal-induced　elemental　segregation　process.　The　elements　of　Na,　Sn　and　Se　were　revealed　to　segregate　severely　and　even　porous　regions　appeared　above　350　degrees　C.　Isolated　Na,　Sn　and　Se　islands　were　formed　through　the　heating　processes.　These　observations　revealed　a　possible　performance　degradation　of　hole-doped　SnSe　through　the　applications　in　thermal　cycling　processes.　A　comparison　experiment　carried　out　on　bulk　Na-doped　SnSe　samples　showed　a　much　better　thermal　stability　and　a　stable　electronic　transport　property.　However,　the　current　investigation　and　results　suggest　more　systematic　researches　need　to　be　considered　to　completely　preclude　or　delay　the　nucleation　and　propagation　of　these　defects　and　the　thermal-induced　elemental　segregation　processes.　(C)　2016　Elsevier　B.V.　All　rights　reserved.