Experiments were carried out in a specially designed circulating fluidized bed with a 5.6-m tall, 0.08 m ID downer, in which the maximal the solids flux 600 kg m(-2) s(-1) and the maximal solids holdup 0.14 have been achieved. The results show that single peak exists in the PDD curves of the solids concentration for low-density operation, and a higher platform is present in the PDD curves for the high-density downer. Solids flux is a key factor that affects the solids holdup while particle properties are other important parameter. The heavier/better fluidity particle could achieve the structure still exists in the high-density downer, but radial solids distribution gradually becomes more uniform with downward distance and with the increase of solids flux. (C) 2004 Elsevier B.V. All rights reserved.