Because of the large fraction of ash in bed inventory, the ash formation data, i.e. ash initial size distribution, fragment and attrition characteristic, are of significance to determine the design parameters of a CFB boiler. A laboratory method was developed earlier for determining the coal ash formation characteristics with the ash formation database for six coal types. The behavior of ash particles mainly depends on the mechanical strength and the fragmentation and attrition resistance. Analysis of this ash formation database showed that the distribution of the ash mixture from a narrow coal size fraction is of bimodal nature, with both the fine soft component (earth ash and dust ash) and coarse hard component (stone ash) following the Rosin-Rammler (R-R) distribution. This paper presents a six-parameter model to predict the ash composition and the size distribution of the ash components of a coal boiler. The cumulative size distribution curves of earth ash and stone ash could be calculated by using the model, and the results are in good agreement with the experimental data. (C) 2003 Elsevier B.V All rights reserved.