A purged pressure probe has been developed to trace the change of pressure drop with time in the collapse process, and hence the dense-phase voidage and the gas velocity in the dense phase can be obtained. The dense-phase voidage and the gas velocity in the dense phase were investigated in the bubbling fluidized beds of Group A powders of FCC particles with mean sizes from 26 to 130 mu m The results show that both the average dense-phase voidage and the average gas velocity in the dense phase decrease with increasing initial bed height, and the dense phase becomes highly expanded when FCC particles are smaller than 70 mu m.