Four kinds of spherical materials with diameter between 0.3 mm to 0.6 mm were employed as floating internal in fast bed of cohesive particles to reduce agglomerate size and narrow the size distribution. The fast bed of cohesive particles with floating internals preserves basic fluidizing characteristics of fast fluidization. However, the size of agglomerates in fast beds of all four types of cohesive particles employed in experiments was notablely reduced. The effect of floating internals could be obviously observed only when the fluidizing gas velocity is higher than about 3-4 times of incipient fluidizing velocity of floating internals. This phenomenon may indicate an "active velocity" for floating internals to take effect. Under specific operating conditions and a matching between cohesive powder and floating internal, there will be a maximal agglomerate size that floating internals can break. This suggests a proper startup procedure for fast bed with floating internals to prevent the formation of oversize agglomerates. Methods for estimating the gas velocity at which floating internal take effect and the terminal velocity of floating internals and agglomerates were developed to give operating range of fluidizing gas velocity of fast bed with floating internals. A model for calculating maximal agglomerate size that floating internal can break was also developed, and an analysis on the influence of fast bed's operating conditions on the maximal size was made based on the model. (C) 2008 Published by Elsevier B.V.