Within　the　range　of　pressure　from　12　to　22.5　MPa,　mass　flux　from　170　to　420　kg/(m2·s)　and　inner　heat　flux　from　150　to　366　kW/m2,　an　experiment　was　conducted　to　investigate　on　the　heat　transfer　characteristics　of　vertical　upward　internally　ribbed　tube.　The　experimental　results　show　that　dryout　occurs　at　subcritical　pressure.　With　the　increase　of　inner　wall　heat　flux　and　pressure,　dryout　occurs　ahead　and　the　peak　wall　temperature　after　dryout　goes　up.　The　effect　of　mass　flux　is　non-monotonic　and　there　exists　a　critical　mass　flux.　When　mass　flux　goes　up,　dryout　appears　ahead　if　it　is　lower　than　the　critical　mass　flux　and　is　delayed　if　it　is　above　the　critical　mass　flux.　At　subcritical　pressure　in　near　critical　pressure　region,　departure　from　nucleate　boiling　(DNB)　occurs　at　high　heat　flux.　The　increase　of　inner　wall　heat　flux　and　the　decrease　of　mass　flux　can　delay　the　DNB　and　lead　to　higher　peak　wall　temperature　after　heat　transfer　deterioration.　At　supercritical　pressure　in　near　critical　pressure　region,　the　heat　transfer　performs　well　and　the　heat　transfer　is　enhanced　in　the　pseudo-critical　enthalpy　region.　The　heat　transfer　characteristic　is　similar　to　that　in　subcritical　pressure　region.　©　2015　Chin.　Soc.　for　Elec.　Eng.