Gas-liquid two-phase flow in minichannels and microchannels displays a unique flow pattern called ring film, in which stable waves of relatively large amplitudes appear at seemingly regular intervals and propagate in the flow direction. In this paper, the behaviors of ring waves, which correspond to ring films that appeared in ring film flow and disturbed ring film flow regions, have been investigated experimentally in gas-liquid two-phase flows of nitrogen-distilled water and nitrogen/30wt% ethanol-water solution in a 150-mu m-diameter silica tube to elucidate their generation mechanism and propagation behavior. In order to clarify the existence region and characteristics of ring waves, the flow patterns observed in a microchannel were investigated and flow pattern maps were made. Furthermore, the velocity of the ring wave was also investigated and compared with the gas slug velocity. In these velocity measurements, high-speed video images were taken at 6,000 frames per second and the formation of ring films and the relationship between the wave amplitude and velocity were determined. The results indicate an interfacial instability leading to the formation and growth of ring waves with both low and high wave amplitudes. The wave velocity is correlated to the wave amplitude, with the large amplitude waves moving much faster than the low amplitude waves. As a result, coalescence of large and low amplitude waves has been observed.