This study was conducted to investigate on crystallization condition and its effect on quality of melted slag that was made from incineration ash of domestic waste. By increasing operating temperature, good crystallization was developed and 900~1,000°C was proved optimal. Longer detention time made better crystallization development, and critical detention time was 3 hour. But too a long detention time caused residual stress by mutual interference of long crystals. Cooling rate might affect the quality of crystallization, but below 5°C/min had no difference. For quality and economics, minimum operating conditions could be comparatively low temperature of 800°C and detention time of 1~3 hour from which high Vickers hardness of about 660~720 kgf/mm2 was achieved. It was compared well with crystallized commercial glass which is about 630 kgf/mm2. Thus, optimum crystallization conditions was suggested to be : temperature 900~1000°C, detention time 3 hours, and cooling rate 5°C /min. 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.