An investigation was conducted to find out the reasons for agglomeration in a fluidized bed combustion (FBC) power plant. Two typical coal samples were collected for investigation. The first sample was collected when the plant was operating smoothly, and the second was collected immediately after agglomeration. These two samples were subjected to analysis. It was observed that agglomeration of the bed material in FBC plants takes place when the coal sample contains either too many very fine particles or too many very coarse particles or both in very large proportion. The very fine particles present in the coal have considerable plastic properties (caking and swelling tendency) which cause agglomeration. The coarser particles are denser because of their higher ash content, and their higher density causes an increase in minimum fluidization velocity which, in turn, leads to agglomeration and defluidization. Medium size coal particles are most suitable for combustion in a FBC,