Fracture strength, one of the rheological characteristics of powder, entrainment flux and distribution of bulk density at free board were measured in systems of FCC (d(p =) 65 mu m)-calcium carbonate (d.-O.9 pm) and catalyst (d(p) = 50 mu m)--fumed silica (d(p)=0.04 mu m) to clarify the effect on the entrainment flux and distribution of bulk density of adding Ultrafine powder to the fluidized bed. Fracture strength was stronger, and entrainment flux and bulk density at free board were decreased by adding the ultrafine powder to these systems. The added ultrafine powder attached to the surface of the fine powder during fluidization, rather than remaining separate. This su,ggests that the Liltrafine powder increased the interparticle force and caused the fracture strength to increase. The increase in irterparticle force would also give rise to particle agglorneration, as a result ofwhich the appareni agglomerated particle diameter and terminal velocity would increase, and the entrainnient flux and bulk density at free board Would decrease. The critrainnient flux and bulk density at free board were correlated as a function of the fracture strength for the systems examined here. The results suggest that the entraininent flux and bulk density at free board can be controlled by changing the fracture strength through addition ofultrafine powdei-, and that this method may be effective for industrial use.