Abstract: Experiments on instabilities of steam-water two-phase flow in inclined upward internally bare pipes have been performed on a high-pressure steam-water test loop. Different types of two-phase flow instabilities including pressure drop oscillation (PDO) and density wave oscillation(DWO) were observed. Experiments indicated that PDO happened without upstream compressible volume supplied by a surge tank when the rate of test pipe length and test pipe inner diameter L/d >1200. The effects of system parameters such as system pressure, mass flow rate, heat flux and inlet sub-cooling are significant on both DWO and PWO. The analytical results showed that PDO happened at a lower exit quality and in the positive portion of the ΔP-G curves with both channel flow rates oscillation in phase, DWO happened at higher exit quality and in the negative portion of the ΔP-G curves with both channel flow rates oscillating out of phase. The investigation results indicated that the oscillation characteristics in inclined parallel upward smooth pipes were similar with in vertical parallel upward pipes.