In this paper, the effects of fluidization velocity, bed temperature and fuel feeding rate on the defluidization time and agglomerate fraction in the fluidized bed combustion of rice straw were studied. The fuel ash, necks in agglomerates and coating layers of bed particles were studied by means of the scanning electron microscope, coupled with energy-dispersive spectroscopy (SEM/EDS). Results showed that the stickiness of bed particles induced by coating layers is the direct reason for bed defluidization. The alkali metals such as K and Na mainly exist in the outer layer of rice straw particles. During combustion the high temperature can cause the alkali species melting and coating the surfaces of ash particles. Consequently, ash particles become sticky and tend to adhere to the surfaces of bed particles. The large-sized ash particles may act as the necks in the formation of agglomerates. The small-sized ash particles may contribute to the formation of coating layers. (C) 2009 Elsevier Ltd. All rights reserved.