Abstract: Local distribution of the interfacial area concentration (IAC) and void fraction of air-water two-phase flow in a vertical pipe were investigated by using the measurement of optical fiber probe. The inner diameter of the circular pipe was 100 mm, and superficial gas and liquid velocities ranged from 0 to 0.1 m/s and from 0 to 1.0 m/s, accordingly. It is found that the bubble passing through frequency dominates the radial distribution profile of IAC. The axial interfacial area concentration was calculated based on the interfacial area transport model of Ishii-Kim. In the mean time, the analysis of four bubble interaction mechanisms determining the change of IAC shows that it is the local pressure inside the pipe that dominates the value of IAC in the axial direction. A new correlation including the effect of pressure was given for calculating the axial IAC.