Abstract: A way of measuring the interfacial area concentration (IAC) of air-water two-phase flow in a vertical pipe with inner diameter of 50 mm by optical fiber probe was researched. The superficial velocities of gas and liquid ranged from 0.01 to 1 m/s and from 0 to 1 m/s, respectively. The accuracy of optical fiber probe method was evaluated and calibrated by comparing with photographic data from high-speed videos. The radial profiles of IAC were given. Several models of calculating IAC were evaluated against experimental results. In the meantime, on the basis of the variable density model of void fraction, a new correlation of the mean IAC related to central void fraction was given. It is verified that the new correlation is of high accuracy by comparing with the other experimental data.