Abstract: An optical fiber probe is used to study the cross-section of oil-gas-water three-phase slug flow in verti- cal tubes. The result shows that short-time-averaging void fraction is effective method on flow pattern recognition. Meanwhile, by changing the ratio of oil to water in the three-phase flow system, its influence on averaging cross-section liquid holdup is found out. The averaging liquid lengths and averaging cross-sec-tion liquid holdup are obtained in each case.