Crystallization is a significant challenge for the rheological properties of the liquid slag flowing down along the wall of entrained flow gasifier. Viscosity can increase sharply when certain crystals occur inside the slag. Chemical compositions are the intrinsic factors in determining the crystallization characteristics. In former studies, we have investigated the influence of major components such as SiO2, Al2O3, CaO, Fe2O3 as well as atmosphere based on synthetic coal slags. While for MgO, its ratio is less than the above four but higher than the other minors such as Na2O, TiO2. Therefore, in order to get a comprehensive understanding of crystallization characteristics of coal slag, the influence of MgO is investigated by four synthetic slags with different MgO ratios (0%, 4%, 8%, 12%). The experimental methods: differential scanning calorimetry (DSC), single high-temperature thermocouple system (SHTT), water quenching and calculation method -FactSage were combined to study the crystallization temperature, morphology of crystals as well as crystalline phases. The final state of slag with no MgO is almost glass with little crystals. The increase of MgO can lead to an enhancement of crystallization temperature and move the temperature-time transformation curve to high temperature region. Mg2+ ions acting as the network modifier participate actively into the crystallization process. At the temperature higher than 1200 degrees C, Mg2+ exists in spinel and the liquid. While as temperature decreases below 1200 degrees C, all the Mg2+ precipitates from the slag forming magnesium-silicate minerals such as diopside and melilite.