The effects of operating parameters such as fluidizing gas velocity U. and total solid inventory M, on the performance of the external loop composed by the loop seal and standpipe in a circulating fluidized bed (CFB) system were often ignored in the existing studies and the limited data are still in controversy. In this paper, the characteristics of gas-solid flow in the external loop was studied in a CFB cold test rig with a square riser of 0.1 in x 0.1 in in cross-section and 3.2 in in height. The results show that solid circulating rate G(s) in the external loop is a function of the operating parameters such as M-t, U-g and aeration rate. An empirical equation was proposed for the G, in the external loop with the operating parameters in the riser. The pressure drop across the loop seal is directly proportional to G, while is inversely proportional to the aeration rate. The pressure drop gradient, voidage and solid height in the standpipe are also influenced by the operating conditions. The controversy among the published literature was discussed and clarified. The results of present study indicated that the external loop cannot be isolated from the entire CFB system for quantitative performance evaluation. Crown Copyright (c) 2008 Published by Elsevier B.V. All rights reserved.