Effects of impeller blade number number on gas dispersion and mass transfer rate were thoroughly investigated for mechanically agitated vessels equipped with 2-,4-,6- and 8-straight blades disk turbine impellers. The results show that under the same rotational speed, the impeller with more blades always can disperse gas more effectively, which induces a higher value of La>. However, with the same total power consumption, the 4- blade impeller can obtain a higher La> value than the 6- and 8-blade impellers under a lower gassing rate condition (Q_g 0.5 vvm), but if Q_g exceeds 0.5 vvm, the 6-blade impeller will perform better than the 4- and 8- blade impellers. To examine the results obtained from the single impeller systems, the same approach is applied to measure La> values for the triple stage 6-blade impeller system(3×6) and quadruple stage 4-blade impeller system(4×4). From the experimental results, it can be found that the 4×4 system gives higher La> value than the 3×6 system under gas completely dispersed conditions. By correlating La> with n_b, N and V_s, the following correlation can be given as: La>=0.00119n_b^0.62N^1.56V_s^0.4 or La>=0.0297n_b^0.1(P_g/V)^0.34V_s^0.48 These two correlations can also be used to evaluate the mass transfer coefficient of each impeller region for the multiple impeller systems and the deviation is always less than 10%.