The detailed distributions of solids holdup in an 18 m high-density circulating fluidized bed are mapped under various operating conditions with fluid catalytic cracking particles. Solids flux (G(s)) is as high as 1000 kg/(m(2) s) when the superficial gas velocity (U-g) ranges from 5 to 9 m/s and high-density operation is achieved. For axial distribution, solids holdup decreases exponentially along the riser with a solids holdup up to 0.17 at the riser exist, indicating the high-density operation has been achieved when G(s) > 800 kg/(m(2) s). For radial distribution, a parabolic radial profile at high-density conditions also replaces the "core-annual" flow structure at low-density conditions. Particles can be accelerated to the axial height of 12 m under G(s) > 800 kg/(m(2) s). Both increasing U-g and/or decreasing G(s) can enhance flow development at high-density conditions. The solids-to-air ratio is almost the same at about 70 when high-density operations are achieved.