Abstract: A feasible Laser Doppler Velocimetry technique based on Amplitude Discrimmation has been used to determine the local gas and solids velocity simultaneously, which can lead to the local slip velocity between the gas and solids in a cocurrent downflow circulating fluidized bed (CDCFB).The experimental and analytical results show that the highest local slip velocity is many times greater than the slip velocity of single particle, and the pole position of the slip velocity profile is in correspondence with that of the particle concentration. It is suggested that there should be obvious particle aggregation phenomenon in CDCFB as well as in a riser.