An experimental investigation into vertical upward air-water two-phase flow was performed using a smooth tube and three spirally grooved tubes. All the test tubes are about 13 mm in inner diameter and 8m in length. The experimental data on each test section include pressure drop, radial distribution of void fraction, cross-sectional mean void fraction, and mean film thickness in annular flow regime. The summary of the results is as folows : (1) The transition boundaries of flow patterns are very similar to those of smooth tube. (2) The pressure gradients do not always depend on the magnitude of relative roughness of test tube, but a ribbed tube, having the greatest relative roughness, shows the greatest values in pressure gradient. (3) The variation of grooves produces remarkable difference in radial distribution of void fractions. (4) Mean film thickness for grooved tubes become definitely greater than for smooth tube.