Steam Power Plant, Steam Power Plant is a power generation system with relatively low operating costs because the working medium uses air. Boiler is a device that functions to convert air into superheated steam which has high temperature and pressure. One of the obstacles that occur at Power Steam Plant itself is a decrease in boiler performance caused by the slagging phenomenon that forms on the walls of the boiler. This phenomenon arose because of the replacement of coal carried out by power steam plant. This practical work report describes the handling of the slagging phenomenon in the boiler by using additive coal injection in the boiler. The Coal Additive used is based on Magnesium, which is useful for increasing the AFT value, because it can weaken the inter-particles so that the particles that have reached AFT will turn into dry ash. There are two parameters taken for the successful use of Coal Additives, the first is clinker hardness which is the result of combustion in the boiler. Furthermore, for additional parameters, it is seen from the value of NPHR (Net Plant Heat Rate) which is an indicator of the reliability and efficiency of a thermal generator, especially power steam plant. The smaller the NPHR value, the more efficient it is.