Abstract: An experimental study was carried out on the pressure-drop type instabilities of the two-phase flow of the parallel vertical rifled tube in the high-pressure vapor-water test loop.The comparision of the effects of two types of tubes on the two-phase flow instabilities have been conducted.The results show that with the increase of inlet pressure and mass flow rate,the boundary heat flux of pressure-drop type oscillations increases;the boundary dryness thresholds of pressure-drop type oscillations increase with the increase of inlet pressure.In addition,with the increase of mass flow rat,the boundary dryness thresholds of pressure-drop type oscillations are increased with rifled tube of Φ31.8 mm×6 mm,but first increased and then decreased for the rifled tube of Φ28.6 mm×5.8 mm.It is also found that the range of the boundary dryness thresholds and the periods of the oscillations of pressure-drop type oscillations for two types of tubes are similar.Under the same operation parameters,the rifled tube of Φ31.8 mm×6 mm is more stable.