The effect of pressure on gas holdup was investigated experimentally with gas-liquid systems, of nitrogen-water, and carbon dioxide-water, in a bubble column with diameter of 45 mm. Single nozzles of 1.4, and 4.0 mm in diameter were used as gas disperser. When: the 4.0 mm nozzle was used, no effect of pressure on gas holdup was observed. When single nozzle of 1.4 mm with nitrogen as the gas phase, gas holdup increased with pressure. But when carbon dioxide was used as the gas phase, gas holdup showed a maximum at pressure of 0.6 MPa. The gas holdup at 1.1 MPa was approximately the same value as that at atmospheric pressure. To explain these phenomena by the behavior of bubbles in the column, bubble size near the wall and bubble frequency at the center of column were measured.