Sodium-potassium　alloy　(Na-K),　which　is　liquid　at　room　temperature　and　pressure,　is　a　promising　working　fluid　for　high-temperature　heat　pipes.　Although　several　researches　have　been　performed　over　the　past　decades,　the　experimental　data　for　Na-K　thermosyphons,　especially　for　Na-K　(wt.　55%　K),　was　limited　and　needed.　This　study　is　to　use　Na-K　(wt.　55%　K)　as　working　fluid　to　fabricate　a　high-temperature　thermosyphon.　The　outside　wall　temperatures　were　measured　to　estimate　the　heat　transfer　performance　of　the　fabricated　thermosyphon　by　using　10　thermocouples,　which　were　made　of　0.2　mm-diameter　Ni-Cr　and　Ni-Al　wires.　The　thermal　characteristics　of　the　Na-K　thermosyphon　were　studied　at　various　heating　temperatures　(650,　675,　700,　725,　750,　775,　800,　825,　850　and　875　degrees　C),　condenser　lengths　(0.250,　0.220,　and　0.190　m)　and　inclination　angles　(0　degrees　and　50　degrees)　to　determine　the　influence　of　working　conditions.　Those　factors　considerably　affected　the　heat　transfer　performance　of　the　Na-K　thermosyphon.