An experimental study on the interface of the core-annulus flow in the riser of a circulating fluidized bed has been carried out in a lab-scale circulating fluidized bed. The dual-stem differential probe, which was developed for the measurement of differential pressure along axial direction in a dense, vertical gas-solids pipe how has been employed in the experiment. The solid cluster velocity in the riser was obtained statistically by applying cross-correlation analysis to the pressure drop signals obtained from two probes. The interface of the core-annulus flow inside a riser was revealed by the profiles of the solid cluster velocity. The experimental study also indicated that the interface of the core-annulus flow in the riser of a circulating fluidized bed varied with the height above the distributor and the operating conditions. The high solids circulation rate led to a thicker, downward moving particle layer near the wall of the pipe. An S-shaped profile of the core-annulus interface along the riser was observed in the experiments.