A new fluidization regime, the circulating fluidization regime, is identified for the upwards liquid-solid fluidization operation. This circulating fluidization regime is characterized by non-uniform radial distribution of liquid velocity, particle velocity and particle hold-up in the bed, different from the conventional fluidization regime and the transport regime where the radial flow structure is more uniform. Because of the radial non-uniform flow structure, the flow behaviours in the circulating fluidization regime cannot be predicted with the empirical relationships based upon homogeneous fluidization.