The particle circulation rate of the clapboard-type internal circulating fluidized bed (ICFB) is investigated with the particle tracing method. For quantitative determination, the capacitance probe and the differential pressure sensor are used to measure the voidages in the high and low gas velocity regime. The mechanism governing the solid circulation in the ICFB is the competition between driven force, caused by pressure drop across the gap, and the resistance force of particle flow, derived by the flow friction in the wall surface and clapboard, the partial resistance loss of clearance, and the gas bypassing formed from the high to the low gas velocity regime. A modified La Nauze model helped to predict and to calculate the particle circulation rate without giving data of pressure drops.