To clarify the primary slag melting behavior of sinters, the microstructure and phase changes during the melt producing process were observed: Sinters were heated under the temperature history and the CO/CO₂ ratio simulating a blast furnace condition and quenched from several different temperatures between 1000 and 1250°C. The microstructures of the quenched samples were observed by electron probe microanalyzer. The olivine-like melts have been locally observed as primary melts above 1100°C, the compositions of which have higher FeO content and lower CaO/SiO₂ ratio with an increase in the heating temperature. On the other hand, microstructures composed of round shape wüstite and 2CaO · SiO₂ have been observed on a sample quenched from 1250°C. In that local area, the olivine-like melts cannot be found. This may imply that although the olivine-like melts originally existed between wüstite and 2CaO · SiO₂, the melts have been completely dissolved into 2CaO · SiO₂ owing to the large solubility of olivine in 2CaO · SiO₂, resulting in the resolidification of the melts. The eutectic structure composed of wüstite, 2CaO · SiO₂ and 2CaO · Al₂O₃ · SiO₂ have also been observed on a sample quenched from 1250°C.