The wave motion and breakup of an annular liquid skeet in a gas stream at relatively small velocity were investigated experimentally and theoretically. It was confirmed experimentally that the wave amplitude of the liquid sheet interface grew downstream of the nozzle, finally breaking up into fine liquid drops in various breakup patterns. The wave motion on the gas-liquid interface in the annular liquid sheet and its relation to the atomization phenomena were studied experimentally using optical measurement technique and were studied theoretically based on the Kelvin-Helmholtz instability theory. The effects of the inner and outer gas stream velocities on wave motion and breakup were clarified experimentally. The theoretical results agreed qualitatively with the experimental results on the wave number and the growth rate.