Abstract: Gas-liquid slug flow is widely encountered in many practical industrial applications. A detailed understanding of the hydrodynamics of the liquid film surrounding gas slug has important significance for modeling of the slug flow. Non-intrusive flow visualization using a high speed video camera system was applied to study liquid film characteristics of slug flow in a vertical narrow rectangular channel (3.25 mm×40 mm). The liquid film thicknesses on both left and right narrow sides surrounding gas slug are different and wavy, but it has a weak effect on liquid film velocity and the velocities on two sides are equal. The gas and liquid velocity as well as the length of gas slug have a significant effect on the liquid film breaking away thickness. The liquid film breaking away velocity increases with liquid superficial velocity, and decreases with the increase of gas superficial velocity at low liquid velocity. The film does not descend and the gas superficial velocity has no significant effect on liquid film breaking away velocity at high liquid velocity (j_L≥1.204 m/s), it is mainly determined by the liquid flow rate.