Ash-related problems are often the main reason for entrained-flow gasifiers and boilers unscheduled shut downs. Thus, understanding the high-temperature physical properties of molten coal ash slag may enable an accurate description of the condition of ash deposition in the gasifier and boiler. The evolution of the time dependence of rheological behaviors, including the viscosity and yield stress of the molten coal ash slag containing 0-35.51 vol % crystals at decreasing temperatures (1400-1280 degrees C) and within 1.05 x 10(5) s, were investigated. Non-Newtonian behaviors, including shear thinning, become obvious with an increasing crystal contents. The trends in the change of crystal nucleation rate in molten coal ash slag are similar with those of the growth rate. Finally, the experimental results and the crystal-size distribution method are used to develop a semiempirical parameterization that describes the complex non-Newtonian rheology of crystal-bearing molten coal ash slag. (c) 2013 American Institute of Chemical Engineers AIChE J, 59: 2726-2742, 2013