As regards the incineration process of the urban solid waste, the composition correct management allows not only the valorization of precise civil and industrial groups of waste as alternative fuels but also a considerable increase of the furnace work temperature leading to a remarkable improvement of the related energy efficiency. In this sense, the study of the melting behavior of ashes deriving from several kinds of fuels that have to be processed to heat treatment is really important. This approach, indeed, ensures to know in depth the features defining the melting behavior of these analyzed samples, and as a consequence, gives us the necessary data in order to identify the best mixture of components to be incinerated as a function of the specific working temperatures of the power plant. Firstly, this study aims to find a way to establish the softening and melting temperatures of the ashes because they are those parameters that strongly influence the use of fuels. For this reason, in this work, the fusibility of waste-derived ashes with different composition has been investigated by means of the heating microscope. This instrument is fundamental to prove the strict dependence of the ashes fusion temperature on the heating rate that the samples experienced during the thermal cycle. In addition, in this work, another technological feature of the instrument has been used allowing to set an instantaneous heating directly on the sample in order to accurately reproduce the industrial conditions which characterize the incineration plants. The comparison between the final results shows that, in effect, the achievement of the best performances of the furnace is due to the a priori study of the melting behavior of the single available components.