In this paper are presented the results of experiments focused on the study of thermal treatment influence of selected black coal flyashes from the heating plant in Kosice and the power plant in Vojany. The study was realized with original not pretreated samples.The obtained results confirmed that after the thermal treatment of both samples the phase’s change of material occurred. At 1050 °C,the decrease of amorphous phase was remarked, being transformed to the mullite and spinel. This information allow of the use examinedfly ashes samples as the matrix for the mullite composites preparation providing the stoichiometric change of thermally treated mixture.