Abstract: Gas-liquid slug flow is widely encountered in many practical industrial applications. A detailed understanding of the hydrodynamics of the liquid slug has important significance for modeling slug flow. A high speed video camera system is applied to visually study the liquid-slug characteristics of slug flow in a vertical narrow rectangular channel（3.25×40mm2）. It is found that the stable liquid slug in the narrow rectangular channel consists of three regions: wake region of leading slug, recovering region of the velocity profile to main flow, stable main flow region. The wake region of leading slug is resulted from the jet of the wall liquid film in the wake of leading gas slug. The position of trailing gas slug in the liquid slug has significant influence on its characteristics. The leading slug also has enormous effect on the trailing slug in laminar or transition flow. The minimum stable liquid slug length Lmin is referred to the length of axial velocity profile near wall region in the liquid slug approaching to fully developed turbulence region（ReTP＞9000）, Lmin increases with increasing slug length and varies inversely with two-phase Reynolds number, and it is in the range of 9-17Dh.