The　phenomenon　of　negative　frictional　pressure　drop　could　occur　in　vertical　two-phase　flow　in　some　conditions.　At　gas-liquid　ratios　ranging　from　100　to　10,000,　the　two-phase　flow　in　a　liquid-loading　gas　well　was　experimentally　simulated　in　a　vertical　pipe.　The　superficial　gas　velocities　range　from　0.1　m/s　to　20　m/s　and　the　superficial　liquid　velocities　range　from　0.00001　m/s　to　0.2　m/s　corresponding　to　the　gas-liquid　ratios.　The　negative　frictional　pressure　drop　was　observed　in　this　experiment.　The　uncertainty　analysis　shows　that　the　negative　frictional　pressure　drop　could　not　be　undoubtedly　attributed　to　measurement　error.　Further　investigation　indicates　that　the　energy　loss　is　associated　with　a　buoyancy-like　term　in　addition　to　the　frictional　pressure　drop　in　vertical　two-phase　flow.　By　taking　this　buoyancy-like　term　into　account,　the　energy　losses　obtained　from　experimental　data　are　all　positive　in　the　cases　of　negative　frictional　pressure　drops.　The　results　demonstrate　the　potential　for　the　negative　frictional　pressure　drop　in　which　case　the　energy　loss　is　positive　and　hence　the　rule　of　energy　conversion　is　not　violated　in　vertical　two-phase　flow.　(C)　2013　Elsevier　Inc.　All　rights　reserved.