The results of the treatment of fly ash from a municipal solid waste incinerator (MSWI) by melting are described, and the safety and the effectiveness of using the slag produced by this melting treatment are studied. The properties of the MSWI fly ash slag were analyzed, to evaluate the feasibility of its reuse as a substitute for part of the cement required in mortar preparation. This MSWI fly ash slag was found to be comprised mainly of SiO 2 and CaO, which can be substituted for up to 20% of the cement content in mortar, without sacrificing the quality of the resultant concrete. In fact, the concrete thus produced has greater compressive strength, 10% higher than that without the substitution. The setting time of the fresh mortar becomes lengthens as increasing amounts of cement are replaced; while the spread flow value increases with the increasing percentage of cement substitution. X-ray diffraction analysis reveals that when the W/ C=0.38 and the curing age=28 days, the crystal patterns in the mortar samples, prepared with different amounts of cement having been replaced by MSWI fly ash slag are similar. According to the results of the toxic characteristic leaching procedure analysis, MSWI fly ash slag should be classified as general non-hazardous industrial waste, that meets the effluent standard. Therefore, the reuse of MSWI fly ash slag is feasible, and will not result in pollution due to the leaching of heavy metals.