Local transient solids fraction and particle velocity signals were measured in a 0.418m×18m large-scale circulating fluidized bed(CFB) by a dual-optical fiber density probe and a Laser Doppler Velocimeter(LDV) system. The probability density distribution(PDD) of both transient solids fraction and particle velocity was obtained. The bimodal distribution of PDD for both solids fraction and particle velocity can be clearly seen in the riser.While in the downer

the PDD curves only exhibit single peak distribution.The results indicate that two phases

i.e.

the cluster phase with solids fraction near 1-εmf and the void phase with solids fraction about 0.01—0.02

exists in the riser. Meanwhile

only single void phase exists in the downer. Although there exist particle clusters in the downer

they cannot form a stable cluster phase with solids fraction near 1-εmf.The dissimilarity of solids flow structure reveal the significant difference in gas-solids flow structure and mechanism between the along-gravity gas-solids flow in the downer and the against-gravity flow in the riser.