•An in-situ observation was performed to record the crystallization process of a synthetic coal slag.•Temperature Time Transformation (TTT) and Continuous Cooling Transformation (CCT) diagrams of the slag were constructed.•With decreasing temperature or increasing cooling rates, the crystals became finer and smaller.•Based on the classic JMA equation, the crystallization kinetics and mechanism were determined. In slagging gasifiers, the crystallization ratio and crystal morphology are of great importance to fluidity of slag. Although increasing attention has been paid to the influence of crystallization on viscosity, few studies have investigated crystallization kinetics in slag melts due to challenges associated with high temperature and experimental complexity. In order to gain an understanding of crystallization characteristics, an in-situ observation Single Hot Thermocouple Technique (SHTT) system was set up and direct observation of the crystallization process was conducted. After completely melting, the slag would form a film as a result of its surface tension on the tip of the thermocouple. When the slag reached a certain degree of undercooling, crystals would precipitate from the homogenous melt. The crystallization ratio was quantitatively determined by taking advantage of the crystal color difference from the surroundings. A synthetic slag was produced from five oxides, with composition 38.4SiO2–14.8Al2O3–20.8CaO–19.1Fe2O3–6.9MgO, which is modeled on a real Chinese coal ash slag with low liquidus temperature and distinct crystallization characteristics. The crystallization characteristics of this synthetic slag were studied under isothermal temperatures and continuous cooling rates. Temperature Time Transformation (TTT) and Continuous Cooling Transformation (CCT) diagrams of the slag were constructed and a fundamental understanding of crystallization influenced by temperature and cooling rates was obtained. With decreasing temperature or increasing cooling rates, the crystals became finer and smaller. At lower temperatures with high degrees of undercooling the incubation time was shortened and the crystallization ratio increased. The influence of cooling rate was not significant until it exceeded 80°C/min. Then the growth of crystals was greatly suppressed by high cooling rates, even appearing glassy when it surpassed the critical cooling rate. Based on the classic JMA equation, the crystallization kinetics and mechanism were determined. The Avrami parameter n indicates that at temperatures higher than 1200°C, interfacial reaction mainly controlled the crystallization process, while at lower temperatures, diffusion was dominant. The crystals formed in different temperature regions may be different phases, which can also be predicted by Factsage Software, but needs further validation.