Abstract: Radial distributions of interfacial parameters including local void fraction, interfacial area concentration and bubble Sauter diameter were measured in low-pressure subcooled boiling flow of water in a vertical circular tube of 24 mm diameter using the double-sensor optical fiber probe. The experimental results show that the phase distribution in vertical circular tube is symmetrical. With heat flux increasing, the local void fraction varies from wall peaking to intermediate peaking, and core peaking can be seen under high heat flux condition. The local void fraction decreases as mass flux increases, and the phase distribution has a transition from intermediate peaking to wall peaking. Further more, it is also found that the local interfacial parameter decreases as pressure increases.