Effects of surface tension on vertical upward annular air-liquid flows in a 5.0 mm I.D. circular pipe has been studied experimentally using water and a low surface tension water added by a little Polyoxyethylene Lauryl Ether as the test liquid. Experimental data on frictional pressure drop, mean liquid film thickness, liquid droplet fraction, interfacial shear stress, and interfacial wave velocity were obtained. In this paper, from these, the mean liquid film thickness and the liquid droplet fraction data together with some flow pictures are presented to clarify the effects of surface tension. In addition, test results of several correlations in literatures and the present new correlation against the present data are also shown.