The composition of the fly ash is adjusted by SiO2 addition order to fit it into the proper range of basicity. The main components of the ash are CaO, SiO2, Al2O3.The fly ash sample filled in alumina crucible and then melted in electric furnace at 1400℃ for 1hr. By decreasing temperature, the best condition of crystallization was obtained in the range of 1100℃ ~ 1000℃, the longer retention time made the better crystal growth, and critical retention time was 3 hrs. The most effective cooling rate for crystallization was recognized as 5℃ / min. For quality and economics, minimum operating conditions could be comparatively low temperature of 1050℃ and detention time of 1~4 hour from which high Vickers hardness of about 713~743 kgf/mm2 was achieved. Also, observation of microstructure of crystallized slag by SEM identified reinforcement of matrix by particles and crack deflection that could render the crystallized slag an improved mechanical strength. By estimate of chemical stability using the toxicity characteristic leaching procedure (TCLP) method of U.S., the crystallized slag showed an excellent resistance against leaching of heavy metal ions in water. Mineral in crystallized slag was identified as wollastonite.