This paper describes the slagging problems resulted from the coal chemistry in various coal-fired electrical power generation plants in Turkey. The fusion temperature test has been used to evaluate the mAng and slagging behaviour of coal ash for years. However, the coal ash fusion temperatures alone are not sufficient to be able to separate the good versus poor performance of coals. Further investigations about the subject revealed that the iron levels in the coal ash and several indexes such as base to acid ratio, slagging factor etc. are also providing good bases for separating the coals that caused problems. The studies using different approaches showed that the chemical and mineral compositions of coal ash determine its melting characteristics and fusion temperature. The correlative equations based on this assumption are still the most widely used techniques for assessing the deposition characteristics and fusion temperature. It also involves the investigation of chemical and mineralogical composition of coal ash samples from Kemerkoy-Yenikoy-Yatagan-Soma coal-fired electrical power plants. The effects of slag indexes causing slagging were calculated after the determination of the ash fusion temperatures. The calculated values were compared with chemical compositions. The results showed that Soma-Yatagan-Yenikoy coal fly ash holds a tendency of high capacity slagging. Kemerkoy coal ash samples have less slagging tendency compared to the others. This is most probably due to their high acidic alkali content.