The nature of fluidization and the behavior of particles in a centrifugal field are discussed Powders are classified into four groups (A, B, C, and D) following Geldart based on their fluidization behavior A simple general model, agreeing with conventional fluidized beds and usable for variable "g," is proposed to determine the transition boundary between group A and C particles. The general model for conventional fluidized beds by Foscolo and Gibiloro was used to determine the boundary between group A and B particles, and the semiempirical equation of Molerus for the boundary between group B and D pal-tides in a centrifugal field. Theoretical analysis showed that group A particles can shift to group B, and group C particles can shift to group A under a centrifugal force. Therefore, certain group C particles carl be fluidized in rotating fluidized beds. For very high "g," however, such particles shift to group D and cannot be fluidized. Experiments in a horizontal rotating fluidized bed under different "g" forces to check the theoretical analysis confirmed that Geldart group C particles (7 mum alumina), which cannot be fluidized in a conventional fluidized bed can fluidize in a rotating fluidized bed operating at a sufficiently high rotating speed to shift them into group A or B. Similarly, in a series of experiments in a rotating fluidized bed group A particles behaved as group B.