Hydrodynamics and flow development are studied in a long riser circulating fluidized bed reactor (15.1 m). Optic fiber probes were used for the measurement of local solids distribution. Pressure drops were also measured with pressure transducers along the riser. The flow development in the riser center is almost instant with solids holdup remaining constant and low, and particle velocity remaining high along the riser. The particle flow is firstly developed from center, and then towards the wall. The riser height is an important factor for the design of circulating fluidized bed reactors. Increasing the solid circulating rate significantly slows down the flow development process, while increases in the superficial gas velocity accelerate it.