Unburned carbon (UBC) percentage in coal ash can be considered as an economic and environmental constraint since it is not only an important characteristic in terms of combustion efficiency but also it becomes more of an issue of further utilization of ash. In this study, particle size distribution (PSD) effect on UBC in fly ash (FA) was discussed and efficient size parameter of PSD in terms of UBC formation was determined. At the first stage of the study, regarding ash formation, percentage of any set of coal particles which are more likely to form fly ash (FA) was theoretically determined. For this purpose, a formulation including the size parameters was proposed. Secondly, with the help of a 3-day long systematical control of power, the effect of size distribution on UBC was obtained. The size distributions of these samples showed that an increase in D-32 size parameter causes an increase in the amount of UBC as well. Relationships between the amount of UBC and mean values of size parameters (D-10, D-50, D-90, D-32, D-43) were all statistically meaningful for both units of power plant concerned but D-32 size parameter has a better fit among all the size parameters. In summary, D-32 & UBC have a regression coefficient of 0.90 (R-2 similar to 0.9), while other size parameters (D-10, D-50, D-90, D-43) & UBC have regression coefficients of 02 (R-2 similar to 02) for both units. Finally, a model including these size parameters was proposed to predict UBC in FA. The model proposed was in good agreement with the measured UBC in FA. (C) 2015 Elsevier B.V. All rights reserved.