An experimental study has been performed to investigate characteristics of the freezing heat transfer of layered air-water flow in a circular tube, in which cooled air and water co-flow. The experiments were carried out under a variety of conditions of water discharge, water temperature, tube-wall temperature, air discharge, and water level. Particular attention was focused on photographic visual observations of the developing ice layer along the tube wall. It was in general observed that there were two different regions characterizing the ice formation, one of which was the freeze-off region, and the other was the steady-state region. The onset of freeze-off was found to be predicted by the equation Fo=1.85Re_w^0.142Re_a^-0.03θ_c^-1.01(H/D)^1.31. It was also found that the mean Nusselt number might be described as the following equation : Nu_m=2.26×10^-1Re_w^0.80Re_a^-0.03$theta;_c^-0.788(H/D)^0.331.