Experimental　investigation　on　the　frictional　pressure　drop　of　high　pressure　steam-water　two-phase　flows　in　an　optimized　six-head　internally-ribbed　tube　has　been　carried　out　in　Xi＇an　Jiaotong　University,　China.　Two　adiabatic　test　sections　were　utilized,　with　one　installed　horizontally　and　another　installed　vertically.　The　frictional　pressure　drops　of　an　adiabatic　horizontal　smooth　tube　were　also　measured　to　provide　a　data　base　for　comparison.　According　to　experimental　data,　the　two-phase　friction　multiplier　was　calculated　and　its　variations　with　the　pressure,　the　mass　flux　and　the　steam　quality　were　obtained.　By　comparison,　it　was　found　that　the　two-phase　friction　multiplier　of　the　horizontal　internally-ribbed　tube　had　an　obvious　peak　value,　which　was　much　larger　than　that　of　the　horizontal　smooth　tube,　in　the　region　with　high　steam　qualities.　It　was　also　found　that　the　two-phase　friction　multiplier　of　the　horizontal　internally-ribbed　tube　was　always　greater　than　that　of　the　vertically-upward　internally-ribbed　tube.