The agglomeration phenomenon of tobacco stems was investigated on a pilot-scale fluidized bed at temperature between 800℃ and 900℃ with two kinds of bed materials silica sand and cinder, followed by a discussion on the agglomeration mechanism. The results indicated that the inclination of agglomeration for the two bed materials above was different. Severe incineration was found in silica sand which led to defluidization finally, while no agglomeration was discovered during long-term run. The data from X-ray fluorescence and scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) indicated that Al, Fe contained in cinder could react with alkali metals in the tobacco stem ash to form high-melting compounds, which prevented the agglomerates to grow, therefore inhibit the agglomeration.