Batchwise fluidized bed experiments have studied secondary fragmentation of char particles during combustion. Two chars, one from a nonswelling and the other from a swelling coal, have been tested for different particle sizes and oxygen concentrations in inlet gas. Statistical functions expressing the probability of particle fragmentation and size distribution of fragments have been determined. Model calculations show that secondary fragmentation of chars significantly influences the performance of fluidized bed combustors only if it occurs in early stage of combustion and if the fraction of particles subject to breakage is above 75%.