In a companion paper ("Modelling Bubbly Flow by population Balance Technique Part I: Axial Flow Development and Its Transitions, " ASME J. Fluids Eng), a two fluid model along with a multiclass population balance technique has been used to find out comprehensive criteria for the transition from bubbly to slug flow, primarily through a study of axial flow development. Using the same basic model the transition mechanism has been investigated in the present paper covering a wide range of process parameters. Though the dominating rate of bubble coalescence during the axial development of the flow acts as the main cause for the transition to slug flow, the simultaneous transformation of the radial voidage pattern cannot be overlooked. Appearance of core, intermediate, wall, and two peaks are observed in the radial voidage distribution depending on the phase superficial velocities. A map has been developed indicating the boundaries of the above sub-regimes. It has been observed that not only the size of the bubbles entering the inlet plane but also the size distribution (monodispersion or bidispersion) changes the voidage peak and shafts the transition boundary. It is interesting to note that the bubbly/low only with a core peak void distribution transfirms into slug flow with a change in the operating parameters. Transition boundary is also observed to shift with a change in the tube diameter. The simulation results have been compared with experimental data taken from different sources and very good agreements have been noted. [DOI: 10.1115/1.3203206]