A flow visualization apparatus is developed to study high pressure flow phenomena in a gas-liquid bubble column. The apparatus can be operated at a pressure as high as 21 MPa. The flow behaviour in the column can be directly visualized through transparent windows. Particle image velocimetry (PIV) is employed to characterize the bubble flow. The study indicates that pressure affects the hydrodynamics mainly through the variation in bubble size and its distribution. The bubble size decreases, the bubble size distribution narrows, and the gas hold-up increases as the pressure increases. The pressure effect on the Sauter mean bubble diameter is not significant when the pressure is higher than 1.5 MPa. Photographic presentation of bubble flow at high pressures is also given.