One of the fundamental functions of a circulating fluidized-bed reactor is the dense suspension of particles in violent agitation, and the performance of the reactor depends largely on the particle concentration. It is thus necessary to estimate the solid concentration profile for the design of fast fluidization reactors. Although the influence of pressure or the density ratio between fluid and solid on the particle concentration in the circulating fluidized bed has been experimentally investigated, it has not yet been fully understood theoretically. Not a few plant development units now in operation have, however, been designed on empirical experiences supported by basic experiments. The purpose of this paper is to clarify the influence of the fluid/solid density ratio on the vertical concentration profile by application of the Growing Chain Model. It is found that the distance from the riser top to the point where solid concentration attains actually its final value, decreases with increase in the fluid/solid density ratio, thus making the bed more uniform vertically. The final concentration is independent of the fluid/solid density ratio. These findings coincide with some experiments by other investigators.