Effect of the composition of major components on the crystallization behavior of the melt-solidified slag by slow cooling process (2 K·min−1) was studied. In case of the synthetic melt-solidified slag with minor components, crystallization of the slag is progressed by slow cooling process when the basicity (= (%CaO)/(%SiO2)) is higher than 0.8. The precipitated crystalline phase is only Gehlenite (2CaO·Al2O3·SiO2). Crystallization of the slag initiates from the interface of the slag and the crucible, and from the surface of the slag. The basicity higher than 0.9 is required in order to precipitate the crystal phases over the entire slag. The influence of Al2O3 content on the crystallization is very little compared with that of the basicity. On the other hand, in case of SiO2-CaO-Al2O3 slag without minor components, the crystallization is promoted at higher Al2O3 content and lower basicity. Only few amounts of minor components in the slag can influence on the crystallization behavior, and even change the crystallized phase.