Ultrasound Doppler velocimetry (UDV) is used to investigate particle velocity field in a liquid-solid circulating fluidized bed. Radial deviation index is used to scale radial non-uniformity of particle velocity. It is found that particle flow structure shows considerable radial non-uniformity

namely core-annulus flow. At certain axial positions

central particle velocity increases with the increase of superficial liquid velocity and particle circulation rate. In the developed region

central particle velocity and radial non-uniformity of flow structure decrease along the axial position.Radial non-uniformity of particle velocity increases with the increase of superficial liquid velocity and particle circulating rate at certain axial positions. In the entrance region

radial non-uniformity is larger than that of the developed region and central particle velocity is less than that at the beginning of the developed region because of particle acceleration. The influence of the entrance region increases with the increase of superficial liquid velocity.