In the present work we investigated bubble swarm properties and gas-liquid mass transfer characteristics in an acrylic resin bubble column, which equipped with a micro-bubble generator at their. The sphere-equivalent bubble diameters were measured by a digital micro scope which recorded the time-series bubble swarm images in the column at an interval of 1/4000 s. Bubble swarm images were processed on a software for non-overlapping flames to evaluate about 1000 numbers of sphere-equivalent bubble sizes. The Sauter diameters were also determined. Experiments for transient physical absorption of oxygen (or ozonated oxygen) from the micro-bubble swarm were carried out after desorbing by nitrogen in order to evaluate the volumetric liquid-side mass transfer coefficient. Micro-bubble generating patterns were classified into three categories: a micro-bubble region at lower gas velocities, a transition region at intermediate gas velocities and a milli-bubble region at high gas velocities. The bubble size distribution in the micro-bubble region was narrow and presented by a log-normal distribution function. In the transition region, the population of milli-bubbles gradually increased, while that of micro-bubbles significantly decreased and the bubble size distribution became a broader shape. In the milli-bubble region, population of the milli-bubbles became dominant. In the micro-bubble region, the dissolved oxygen (or ozone) concentration increased with a small speed but it finally exceeded the equilibrium value with the gas phase oxygen (or ozone) pressure. On the other hand, in the transition and milli-bubble regions, the dissolved concentration gradually decreased with increasing superficial gas velocity approaching to the normal equilibrium value.