This report presents the results of systematic experiments conducted in a laboratory scale fluidized bed combustor in order to study agglomeration phenomena during firing straw and co-firing straw with coal. The influence of operating conditions on ag-glomeration was investigated. The effect of co-firing straw with coal on agglomeration was also examined. The results show that temperature has the most pronounced effect on the agglomeration tendency. As bed temperature increases, the defluidiza-tion time decreases sharply, which indicates an increasing tendency of agglomera-tion. When co-firing straw with coal, the defluidization time can be extended signifi-cantly. Examination of the agglomerates sampled during combustion by various analytical techniques indicates that the high potassium content in straw is the main cause for the formation of agglomerates. In the combustion process, potassium-containing compounds are prone to remain in the bed and form low melting tem-perature potassium rich ash. The molten ashes coat the surfaces of the bed material, promoting agglomeration and defluidization eventually.Based on a competition between the strengthening adhesive force by sintering of the ash coating and the breaking force induced by bubbles, an engineering model has been developed to describe the agglomeration and defluidization phenomena during combustion of straw. The results from the model are in good agreement with the ex-perimental results. From the experimental observation and theoretical analysis, strategies for minimizing agglomeration problem are proposed.