The paper presents an experimental method for studying the fragmentation of solid fuels in bubbling fluidized bed combustion. The influences of properties of fuel and the bed temperature on the fragmentation process were studied. Degree of fragmentation and fragmentation index of the 8 different types of coal were also determined. A fragmentation of coal plays a significant role during combustion process in fluidized boilers because it accelerates combustion and influences on the particle size distribution of the fluidized bed. Fine, unburned coal fractions, formed during fragmentation, increase the loss of incomplete combustion.