The three dimensional flow structure of dilute gas-solid suspensions in a small-scale circulating fluidized bed (0.200 m riser diameter) was visualized by applying the laser sheet technique. FCC particles were fluidized with sustained solid loading at gas velocities corresponding to the turbulent and the fast fluidization regimes in cases where the solid circulation was sufficient. Three typical shapes of clusters in the core section of the riser were observed. Clusters characterized by a paraboloidal shape heading downward were connected to neighboring clusters at their tail part, forming a three dimensional network structure.