In entrained flow gasifiers, the viscosity of molten slag along the wall is critical for continuous operation. Crystallization inside the slag can cause a sharp increase of viscosity and even blockage of gasifier. Nucleation and crystal growth processes of four coal slags were observed in-situ by an advanced high-resolution confocal scanning laser microscope (CSLM). The crystallization ratio and kinetics of three crystalline slags were calculated. Crystal phases were determined by XRD and morphologies were analyzed by SEM. The effect of crystals on viscosity is also discussed. Anorthite is detected as the primary as well as the main crystalline phase of three slags, although the crystals can be in different shapes such as rod, flate or even needle. Anorthite at high temperature can move around inside the slag because of low viscosity and small crystallization ratio and little influence can be found of the drifting crystals on viscosity. But when the crystallization ratio reaches 15-20%, the viscosity can be severely impacted.