Alkali　metal　heat　pipes　play　significant　role　in　various　high-temperature　engineering　applications　because　of　their　excellent　heat　transfer　capacity.　Inclination　angle　is　one　of　major　factors　which　significantly　affect　start-up　and　heat　transfer　characteristics　especially　for　thermosiphons.　A　sodium-potassium　alloy　(Na-K)　gravity-driven　heat　pipe　(GHP),　in　which　the　content　of　potassium　in　Na-K　is　wt.　55%,　was　fabricated　to　study　the　effect　of　inclination　angle　on　start-up　and　heat　transfer　capacities　of　high-temperature　GHPs.　The　Na-K　GHPs　was　fixed　by　the　adjusting　bracket　in　9　inclination　angles　(0　degrees,　10　degrees,　20　degrees,　30　degrees,　40　degrees,　50　degrees,　60　degrees,　70　degrees　and　80　degrees).　Outside　wall　temperature　was　measured　by　eleven　thermocouples　which　calibrated　by　the　China　Institute　of　Metrology　prior　to　using　them　in　the　experiments.　Results　show　that　inclination　angle　has　a　significant　impact　on　start-up　and　heat　transfer　performances　of　the　Na-K　GHP　because　of　the　impact　of　gravity　on　the　two-phase　flow　inside　the　heat　pipe　and　effective　heating　area　in　the　evaporator.　Start-up　and　heat　transfer　characteristics　are　dramatically　improved　and　temperature　difference　significantly　decreases　as　the　inclination　angle　increases　from　0　degrees　to　50　degrees,　but　slightly　decreases　when　the　inclination　angle　exceeds　60　degrees.