Coal, residues and waste produced by the combustion of the coal contain naturally occurring radionuclides such as U-238, Ra-226, (210)pb, Th-232 and K-40 and trace elements such as Cd, Cr, Pb, Ni and Zn. In this work coal and its combustion residues collected from Yatagan and Yenikoy coal fired thermal power plants (CPPs) in Turkey were studied to determine the concentrations of natural radionuclides and trace elements, and their enrichments factors to better understand the radionuclide concentration processes within the combustion system. In addition, the utilization of coal fly ash as a secondary raw material in building industry was also studied in terms of radiological aspects. Fly ash samples were taken at different stages along the emission control system of the thermal power plants. Activity concentrations of naturally occurring radionuclides were determined with Canberra Broad Energy Germanium (BEGe) detector BE3830-P and ORTEC Soloist PIPS type semiconductor detector. The particle size distribution and trace elements contents were determined in various ash fractions by the laser scattering particle size distribution analyzer and inductively coupled plasma (ICP-OES). From the obtained data, natural radionuclides tend to condense on fly ash with and the activity concentrations increase as the temperature drop in CPPs. Measured Pb-210 and Po-210 concentration varied between 186 +/- 20-1153 +/- 44 Bq kg(-1), and 56 +/- 5-1174 +/- 45 Bq kg(-1), respectively. The highest (210)pb an, Po-210 activity concentrations were determined in fly ash taken from the temporary storage point as 1153 +/- 44 Bq kg(-1), and 1174 +/- 45 Bq kg(-1) respectively. There were significant differences in the activity concentrations of some natural radionuclide and trace elements (Pb and Zn) contents in ash fractions among the sampling point inside both of the plants (ANOVA, p < 0.001). Coal and ash sample analysis showed an increase activity concentration and enrichment factors towards the electrostatic precipitators for both of the power plants. The enrichment factors for Zn follow a similar trend as Pb, increasing in value towards the end of the emission control system. The calculated activity indexes were above 1.0 value for both of the power plants, assuming the utilization of fly ash at 100%. It can be concluded that the reuse of fly ash as a secondary raw material may not be hazardous depending on the percentage of utilization of ash. (C) 2017 Elsevier Ltd. All rights reserved.