Abstract: This paper presents the characteristics of three - phase mixing in terms of the critical impeller speed of complete suspension of solid, gas and solid dispersion state, stirred power, gas - liquid mass transfer coefficient for different suspension concentration and gas volume in liquid by using a new type axial impeller(K4) in mechanically stirred vessels. It is revealed that the critical impeller speed Njsg and stirred power per unit volume Pjsg/ V for incipient complete suspension state increase with increasing gas volume. The aeration coefficent k and aeration factor K represent the intensities of these effects, and usually k= 0.124 ~ 0.580, K = 0.72. The total dispersion process in three - phase system can be defined as two dispersion regions. In the first dispersion region, the gas dispersion ia from flooding to complete dispersion while the solid dispersion is from the state of lift - off to complete suspension, In the second dispersion region the gas and solid concentrations are well distributed in the stirred vessels, which is regarded as pseudo - homogeneous dispersion. The impeller speed index m in the equation of power consumption is close to the index for homogeneous system, usually, the mean speed index m = 2.67. The volumetric mass transfer coefficient KLa increases with incresing gas volume as a result of gas holdup increaing with the increasing of the gas volume. The power index β accounts for the effect of the stirred power on the volumetric mass transfer coefficient KLa and usually β= 0.71 ~0.93.