Differential thermal analyses (DTA) of six slags of different added-CaO content fabricated from municipal waste ash were performed with various heating rates. An endothermic peak caused by glass transition and two exothermic peaks caused by crystallizations were observed for four specimens of low CaO-content. With decreasing the heating rate, the two peaks were superposed on each other. On the other hand, only one crystallization peak was observed for two specimens of high CaO-content. In order to estimate the activation energies of the crystallizations, a modified Kissinger equation was defined by use of the first peak in the derivative of the DTA curve. The obtained activation energies were correlated with basicities of the specimens defined by the JIS, ASTM and Ca/Si. The phases of thoroughly crystallized specimens were identified by X-ray powder diffractometry. Pseudowollastnite, anorthite and albite were main crystalline phases for four specimens of low CaO-content, while gehlenite was only phase for two specimens of high CaO-content. These phases could be explained by the SiO₂-Al₂O₃-CaO ternary phase diagram. Through the analysis of the four partially crystallized low CaO-content specimens, it was found that the phase crystallized initially in the two lower CaO-content specimens was pseudowollastonite, and that of the two higher CaO-content specimens was gehlenite.