A unique phosphor tracer technique was used to allow experimental measurement of residence time distribution (RTD) of solids in the circulating fluidized bed. The instantaneous input of light impulse and the detection of the light emission by the excited phosphor particles permit on-line continuous determination of solids tracer concentration with time at different radial and axial positions within the riser and downer. The measured RTD curves in the downer show that less axial solids mixing takes place in the downer than that in the riser. The two-peak distribution of RTD curves in the riser is clearly seen in the experiment.