At　low　mass　fluxes,　experimental　investigation　on　the　frictional　pressure　drops　of　high　pressure　steam　water　two-phase　flows　in　an　adiabatic　horizontal　Siemens　AG　optimized　internally-ribbed　tube　with　a　diameter　of　φ　31.8　mm　×　6.0　mm　was　carried　out.　The　two-phase　frictional　pressure　drops　of　an　adiabatic　horizontal　smooth　tube　with　a　diameter　of　φ31.8　mm　×　6.0　mm　were　also　measured　under　the　same　experimental　conditions　to　provide　a　data　base　for　comparison.　In　addition,　the　frictional　pressure　drops　at　supercritical　pressures　in　the　internally-ribbed　tube　were　obtained.　According　to　experimental　data,　the　two-phase　flow　friction　multiplier　was　calculated　and　its　variations　with　the　pressure,　the　mass　flux　and　the　steam　quality　were　obtained.　At　supercritical　pressures,　the　variations　of　the　frictional　pressure　drops　with　the　mass　flux,　the　bulk　enthalpy　and　the　pressure　were　also　analyzed.　By　comparison,　it　is　found　that　the　two-phase　friction　multiplier　of　the　horizontal　internally-ribbed　tube　has　an　obvious　peak　value,　which　is　much　larger　than　that　of　the　horizontal　smooth　tube,　in　the　region　with　high　steam　qualities.　Comparing　to　the　single-phase　frictional　factor　at　subcritical　pressures,　the　characteristic　of　frictional　factor　at　supercritical　pressures　is　that　there　is　a　sudden　change　at　the　point　of　the　pseudo-critical　temperature,　that　is　to　say,　there　is　a　peak　value.　©2015　Chin.　Soc.　for　Elec.　Eng.