The characteristics of the Smith turbine impeller were studied both experimentally and numerically, and the results were compared to those of the Rushton turbine impeller. The gas recirculation rate around the impeller was examined and the structure of ventilated cavities behind blade as well as the values of forces related to gas dispersion was calculated through the CFD simulation. In addition, the sizes and distributions of dispersed bubbles were determined experimentally at various locations around ventilated cavities to examine the gas dispersion mechanism. Finally, the gassed power and local mass transfer coefficient for both impellers were measured and the results were related to the cavity structure. Both the results after the CFD simulation and bubble size measurements show that most of the total gas is dispersed at the cavity tail. For the Smith turbine impeller, the small-vortex pair attaching to the leading blade makes an additional contribution to gas dispersion. Although the gassed power drawn by the Smith turbine impeller decreases slightly with the increase in aeration rate, the trend is much flatter and tends to be leveling off at a much smaller aeration number as compare to the Rushton turbine impeller. As the aeration number N-A>0.03, the cavity structure of the Rushton turbine impeller changes into the large cavity, while the vortex cavity still clings to the Smith turbine impeller. This fact indicates that the Smith turbine impeller has a better gas handling capability under a higher gassing condition and provides a better mass transfer performance.