The developments of bubble flow structures and their effect on the propagation properties of void fraction waves are experimentally investigated in vertical upwards, air-water flow. The bubble-to-slug flow regime transition (BSFRT) is investigated based on the instability of the void fraction waves. A series of tests are performed by systematically varying the bubble size for different flow conditions. Several statistical parameters are evaluated from the void fraction signals to objectively characterize the developing flow structures and to investigate the wave propagation properties. In particular, the degree of spatial growth of the void fraction waves is quantified in terms of the spatial attenuation factor (SAF). Two distinct modes of flow structural developments in bubbly flow are observed which are dependent on the bubble size. It is also shown that the different features of wave propagation properties are mainly due to the differences in the developing mode of the flow structures. It turns out that BSFRT is associated with both the instability of void fraction waves and the abrupt changes of the apparent time scale, and the neutral stability condition of the void fraction waves corresponds to the appearance of large structures of the gas phase.