Abstract: The vortex geometry and the radius of the cylindrical rotating zone in a vertical self-induction gas-liquid vessel with co-axial propeller agitation was formulated by the momentum balance method and verified by experimental investigations. From this, the critical inductive speed N and the variation of the cylindrical rotating zone with Reynolds number could be predicted. It was found that the propeller performed much better than other types of impellers such as flat blade impeller, flat blade turbine and flat blade disc turbine, because of its lower critical inductive speed and less effect of cylindrically rotating zone. Thus, it was claimed that such a propeller may be a good component for gas-liquid polymerization reactors.