We measured gas holdup, bubble size distribution, volume-surface mean diameter, and specific surface area for oxygen micro-bubbles issuing from a high-speed rotation and compression-dissolution-type micro-bubble generator into a bubble column with a 150 mm i.d. and a height of 1000 mm containing pure water saturated with oxygen at 25.0 degrees C in up-flow and down-flow operations. We also measured the apparent volumetric mass transfer coefficient, k(L)a, in the micro-bubble column using the transient physical absorption of oxygen. The dependence of the values of k(L)a on the superficial gas velocity was interpreted and correlated by a complete absorption model. Furthermore, the oxygen absorption efficiency, which was defined as the percentage of the ratio of the absorbed amount of oxygen to the introduced amount of oxygen, was found to be within the range of 40-98%, decreasing with increasing volume-surface mean diameter. The differences in the physical and mass transfer properties between the up-flow and down-flow operations were also compared and interpreted. The values of the gas holdup and the volume-surface mean diameter in the up-flow mode were greater than those in the down-flow mode at the same superficial gas velocity. On the other hand, the values of the specific surface area, those of k(L)a, and those of oxygen absorption efficiency for the down-flow operation were greater than those for the up-flow operation.