A dual-optic density probe was used to investigate the transient signals of local solids fraction at different radial positions on an axial section in a 186 mm diameter (ID) gas-solids fluidized bed, which was operated under the conditions of superficial gas velocity of 0.025-10.2 m/s with 75 mum FCC particles. The analysis of probability density function (PBF) of the density signal shows that two kinds of stable phase structures exist in bubbling, turbulent, circulating, and pneumatic transport fluidized beds. The dilute phase structure is termed "void", characterized by; a continuous gaseous phase with a log-normal solids fraction distribution. The dense structure is termed "cluster", characterized by a continuous solids phase with a normal solids fraction distribution. The interactions between the two phases in the space scale and the time scale form the macroflow phenomena. The properties of phase structures are uniform over the cross section, and change with the operating conditions. Based on the characteristics of voids and clusters and the interactions between them, a nzw explanation of the mechanism of gas-solids fluidization through four regimes is proposed.