The motion of a single measure of a gas slug in a vertical tube using a closed loop filled with stagnant liquid was studied to investigate the influence of the system pressure under a wide range from near atmospheric pressure to highly elevated pressure, e.g., 20MPa. One purpose of this study was to investigate the influence of pressure on the rising velocity of the gas slug. The experimental equation was presented for rising velocity of the gas slug. The other purpose was to study the phenomenon of the liquid film around the gas slug as pressure increased. It was found that when liquid waves with liquid film around the gas slug appear, there are many bubbles with small diameters in the wake of the gas slug. The correlation of smooth liquid film length around the gas slug was presented.