Abstract: Visualized investigation of the air-water two-phase slug flow in vertical narrow rectangular channel (3.25 mm×40 mm) was carried out with the help of a high speed video camera. The work pressure is atmospheric pressure and the superficial velocities of air and water cover 0.1-2.51 m/s and 0.16-2.62 m/s, respectively. The slug flow in narrow rectangular channel is found to be much different from that in circular pipe. Most of the slug bubbles are deformed, and much more seriously at high liquid velocity. The void fraction of narrow side liquid film is larger than that of wide side liquid film, and the film does not descend at high liquid velocity. It is observed that some tiny bubbles in the areas of the nose and tail of a slug bubble are injected into the liquid film along the wide side. The nose shape of the slug bubble as well as its width has significant effect on its velocity, whereas the effect of the length is very weak. The Ishii & Jones-Zuber drift flux model can predict slug velocity well, except at low liquid superficial velocity by the reason of that the calculated drift velocity is less than experimental values.