The operation of a loop seal in a circulating fluidized bed is studied on the basis of pressure balance of the circulation loop. The sharp-crested theory of free surface flow is applied to analyze the solids flow rate through a loop seal. Factors, which influence the solids flow rate through the loop seal, include loop seal air velocity, initial bed inventory, standpipe size, loop seal slit size and particle size. The solids flow could occur only between two limiting values of each of those parameters. The analysis also presented pressure distributions along the loop for different circulation rates. Results from above theoretical analyses were compared with experimental results. A good agreement between the results confirmed the validity of the present analysis.